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How would I let 1 .aspx page use another .aspx page's code?

I have 2 aspx files in my project. The first.aspx page has some content on it and when I click on a button, it will launch a frame (second.aspx that only has code to show a calendar) on the same page.
Now once that calendar(second.aspx) loads on first.aspx, I want to click a link on the calendar that will .show() a hidden DIV on the first.aspx page.
How do I access code cross pages? In other words, how can I write some code in second.aspx that will affect first.aspx.

What you're asking for is not really possible. You're probably approaching it the wrong way. What you should do is turn your calendar page into a user control so that it can be used seamlessly in first.aspx.
Here is how to get started with user controls in asp.net:
After you turn it into a user control there are different approaches to getting access to the properties of the user control from your page. Here is one approach using the FindControl method.
Hope that helps.

The easiest solution would be to show and hide your div with jquery. Simple give your div a class like:
<div class="myCalendarDiv" style="display:none" />
And your Button should look like this:
<asp:Button id="myButton" OnClientClick="return ShowCalendar();" runat="server" />
<script type="text/javascript">
function ShowCalendar() {
$(".myCalendarDiv").show();
return false;
}
</script>
Another way would be instead of creating a seprate webpage for the calendar, as proposed you can use a jquery dialog, or make a usercontrol and embedd it on the same page.

(Posted on behalf of the OP).
So since I was dealing with an Iframe, I found out that you can target the parent window which would be first.aspx.
I used "window.parent.MYFUNCTION();" to call my JavaScript function on first.aspx and show the div.

Invalid ViewState when using jQuery tabs

I have a fairly simple page with a set of jQuery tabs, the content of some is called via ajax. I also have a search box in the masterpage in my header.
When I open the tabbed page the search box works fine. However once I have clicked on one of the ajax tabs the search box fails to work with an "Invalid Viewstate" yellow screen of death.
I believe this is because the ajax page is replacing the __VIEWSTATE hidden input with its own.
How can I stop this behaviour?
UPDATE: I have noticed that the YSOD only appears in IE and Chrome, Firefox doesn't seem to have the same issue. Although how the browser influences the ViewState, I'm not sure.
UPDATE: I've put a cut down version of the site that shows the issue here: http://dropbox.com/s/7wqgjqqdorgp958/stackoverflow.zip

The reason of such behavior is that you getting content of the ajaxTab.aspx page asynchronously and paste it into another aspx page. So you getting two instances of hidden fields with __VIEWSTATE name and when page posted back to server theirs values are mixing (might depends on how browser process multiple controls with same name on submit). To resolve this you can put second tab's content into a frame:
<div id="tabs">
<ul>
<li>Default Tab</li>
<li>ajax Content</li>
</ul>
<div id="tabs-1">
<p>
To replicate the error:
<ul>
<li>First use the search box top right to search to prove that code is ok</li>
<li>Then click the second ajax tab, and search again.</li>
<li>N.B. Chrome / IE give a state error, Firefox does not</li>
</ul>
</p>
</div>
<iframe id="tabs-2" src="ajaxTab.aspx" style="width:100%;" ></iframe>
</div>
Also, I'm not sure but this seems like error in the Web_UserControls_search control. In my opinion, NavBarSearchItemNoSearchItem_OnClick method must be refactored as below:
protected void NavBarSearchItemNoSearchItem_OnClick(object sender, EventArgs e)
{
var searchFieldTbx = NavBarSearchItemNo;
var navBarSearchCatHiddenField = NavBarSearchCatHiddenField;
var term = searchFieldTbx != null ? searchFieldTbx.Text : "";
if (term.Length > 0) //There is actually something in the input box we can work with
{
//Response.Redirect(Url.GetUrl("SearchResults", term));
Response.Redirect(ResolveClientUrl("~/Web/SearchResults.aspx?term=" + term + "&cat=" + navBarSearchCatHiddenField.Value));
}
}
Draw attention that we resolving client url when redirecting to search results page and instead of navBarSearchCatHiddenField use navBarSearchCatHiddenField.Value as cat parameter.

I guess that you use AJAX to fill the content of the tab. So in this case, content of your tab will be replaced by the new one from ajax and certainly _VIEWSTATE will be replaced. At server, do you use data from ViewState? In the "static tabs", you should prevent them auto reload by using cache:true

Your issue is that with your ajax call you bring in a complete ASPX page. Including the Form tag and its Viewstate. If you remove the Form tag from ajaxTab.aspx you will see everything works fine. asp.net does not know how to handle two Form tags in one page. Same goes for hidden Viewstate fields. You cannot bring in a full aspx page via ajax. Just bring in the content Div you want to display and you`ll be good to go.

ASP.net user control doesn't call a javascript function

I am developing asp.net web application, I have a repeater that call a registered user control, I have in the user control a button that I want to call a javascript function that make Ajax call to do some action on server. this button doesn't call the javascript method, I don't know why? and when I view source I found the javascript function is repeated for every item in the repeater, how to eliminate this repetition specially that I read server items inside the function, and why the function is not called?
Thanks a lot!
sercontrol.ascx
<div id="divBtnEvent" runat="server">
<input type="button" id="btnAddEvent" class="ok-green" onclick="saveEvent();" />
</div>
<script type="text/javascript">
function saveEvent()
{
var eventText = document.getElementById('<%=txtEventDescription.ClientID%>').value;
// make ajax call
}

Problem 1: In view source I found the javascript function is repeated for every item in the repeater.
Solution:
Put your js function on the page on which the user control is being called. You also have to place your js files references on your page not on user control.
Problem 2: You are trying to get control's value as <%=txtEventDescription.ClientID%>.
And I think, this control is on user control.
Solution: Please check your page source code and see that the control's actual clientid is.
If still have issues in calling js function, check firefox's Error consol.
Hope this help.

OP said and when I view source I found the javascript function is repeated for every item in the repeater
to get rid of it put ur javascript as follows
<head runat="server">
//heres goes ur js
</head>
I guess that's ur problem

Replace the saveEvent function definition from user control onto the page where the control is used. All the way as I understand, you have use in this function textbox id from that page.
Actually, if you have place javascript block in user control's markup it will be rendered along with the rest control's markup. To avoid this you may register javascript from the server code and check is that code block is already registered.
By the way, as it is - only the last function definition being taked into attention as it redefined the previous one each time new control instance rendered.

Allow for "<<" to be submitted, without disabling "validaterequest"

I'm wondering if anyone knows of a way to allow something like "<<" to be submitted, without setting validaterequest=false
I have a creole parser, and the recommended plugin/macro syntax is:
<<macro-name argo0=foo arg1=bar argN=qux>>

I wrote a little ‘encodeMyHtml’ JavaScript function that is called on the OnClick event when the HTML form’s submit button is clicked. The function encodes the user’s HTML input for the field I’ve specified into a harmless string before it is passed to the server. When I receive that input on the server I simply decode and go on my way.
ValidateRequest is happy, our users are happy, our peers are happy, heck we’re happy.
I add my ‘encodeMyHtml’ JavaScript function in my user control’s OnPageLoad method. This way I can make sure that my JavaScript is added to the parent page only once, no matter how many controls are on the page.
In my control’s OnPageLoad I call this:
private void addEditorJavaScript()
{
// create our HTML encoder javascript function
// this way it shows up once per page that the control is on
string scr = #"<script type='text/javascript'>function encodeMyHtml(name){
var content = document.getElementById(name).value
content = content.replace(/</g,'<');
content = content.replace(/>/g,'>');
document.getElementById(name).value = content;
}</script>";
// add the javascript into the Page
ClientScriptManager cm = Page.ClientScript;
cm.RegisterClientScriptBlock(this.GetType(), "GlobalJavascript", scr);
}
In my control’s ASPX I’m using a gridview. I wrap the gridview’s update asp:LinkButton in a span tag, and in that span tag I put my OnClickEvent.
<span onclick="encodeMyHtml('<%# UniqueID.Replace("$", "_") %>_FormViewContentManager_ContentTextBox')">
<asp:LinkButton ID="UpdateButton" runat="server" CausesValidation="True" CommandName="Update" Text="[Publish]" />
</span><span onclick="encodeMyHtml('
When I get the input on the server side I simply call a couple of Replace methods on the input string to decode the HTML, and I’m done.

You could do a javascript regex replace for "<\S" on the specific field on form submit. But it would fail for browsers that don't support javascript.

You can encode the "<<" on the client using Javascript:
<script language="javascript">
function encodeString(str) {
return str.replace(/</gi, '<').replace(/>/gi, '>');
}
</script>
And then on the server use Server.HtmlDecode to return the string to its original form.

What does IsPostBack actually mean?

I am interested to know what specifically Page.IsPostBack means. I am fully aware of it's day to day use in a standard ASP.NET page, that it indicates that the user is
submitting data back to the server side. See Page:IsPostBack Property
But given this HTML
<html>
<body>
<form method="post" action="default.aspx">
<input type="submit" value="submit" />
</form>
</body>
</html>
When clicking on the Submit button, the pages Page_Load method is invoked, but the Page.IsPostBack is returning false. I don't want to add runat=server.
How do I tell the difference between the pages first load, and a Request caused by the client hitting submit?
update
I've added in <input type="text" value="aa" name="ctrl" id="ctrl" /> so the Request.Form has an element, and Request.HTTPMethod is POST, but IsPostBack is still false?

Check the Request.Form collection to see if it is non-empty. Only a POST will have data in the Request.Form collection. Of course, if there is no form data then the request is indistinguishable from a GET.
As to the question in your title, IsPostBack is set to true when the request is a POST from a server-side form control. Making your form client-side only, defeats this.

One way to do this is to extend the ASP.NET Page class, "override" the IsPostBack property and let all your pages derive from the extended page.
public class MyPage : Page
{
public new bool IsPostBack
{
get
{
return
Request.Form.Keys.Count > 0 &&
Request.RequestType.Equals("POST", StringComparison.OrdinalIgnoreCase);
}
}
}

In the example that you include in your question, there is no viewstate involved; there is no way for the server to link this request to a previous request of the page and treat them as a unit. The request resulting in clicking the button will look like any other random request coming in to the server.

Generally a you could view a PostBack as a combination of:
HTTP request method equals "POST"
HTTP header HTTP_REFERER equals the current URL
That's not 100% foolproof tho, it does not take into account any state of any kind (which you probably want even if you don't know it) but it is a post, back to the current page.

You could check the headers to see if your input controls are returning a value (using Request.Forms as tvanfosson points out). However, the really big question is why you would not want to add runat=server. The entire page processing edifice implemented by ASP.NET (except MVC) depends on processing the page output through the server to set up the appropriate client-side code for callbacks, etc.