How to display an ASPX in another ASPX's DIV dynamically at runtime?
If you don't want to use the iFrames, you can very well use Object element of HTML. Follow here to see and html example. You can very well use this for aspx also with some change, like using OnClientClick property for aspx button etc.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN""http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"><head><title>mouseover image position</title><meta http-equiv="Content-Type" content="application/xhtml+xml; charset=utf-8" /><style type="text/css">/*<![CDATA[*/body { background-color:#aaaaff; }#one { position:absolute; left:50%; top:50%; margin:-150px 0 0 -250px; }object { width:500px; height:300px; border:solid 1px #000000; } /*//]]>*/</style><script type="text/javascript">//<![CDATA[// written by: Cootheadfunction updateObjectIframe(which){ document.getElementById('one').innerHTML = '<'+'object id="foo" name="foo" type="text/html" data="'+which.href+'"><\/object>';}//]]></script></head><body><div id="one"><object id="foo" name="foo" type="text/html" data="http://www.w3schools.com/"></object></div><div><a href="http://www.google.com" onclick="updateObjectIframe(this); return false;">this is an object test not an iframe test</a></div></body></html>
As far as I know, barring the use of iframes, there is no way to load one aspx page into another.
With postbacks or ajax, you can use UserControls (ascx) instead. They can contain pretty much the same content a page can anyway, or use a MasterPage.
If you wish to have no postbacks, ajax is probably the way to go, though again, it does not allow you to load an aspx page into another, only to change the content of the page you're on (amongst other things).
I'm not sure about other platforms for web development though, they may have a solution closer to what you want to do, so if asp.net is not a "must", you should consider checking out other platforms.
If you're using the AJAX toolkit it should be possible to do this using a webcontrol rather than an ASPX page.
If you try to pursue this idea using ASPX pages, and without using an iframe, you will find that there is no isolation provided for javascript variable names and element ids, therefore almost guaranteeing conflicts if you put the rendered aspx's content into a div using innerHTML; The page will definitely not be able to perform a partial postback as I imagine you would like.
Using a webcontrol instead: A better solution would be to install the AJAX toolkit if you haven't already, and use an updatepanel control. Either dynamically load and unload webcontrols inside this panel (using LoadControl()), or place a Multiview control inside it and change the activeview to simulate changing this content.
The updatepanel will allow its contents to update without a full postback (page refresh).