I have a modal pop extender that has a user control in it, I use it in several forms in a ASP.NET 4.0 project. I am not very happy with the js code that it produces and the mess in the markup :/ So, any worthy alternatives that can be controlled from the codebehind easily :) ?
Having been down the road of Ajax Control Toolkit, I would recommend moving on to jQuery or some other javascript framework. While I understand the desire to "control it from codebehind," because it seems easier at first, trying to wrap client-side functionality in a server-side control usually is more trouble than it's worth.
Even in the simplest case, the the amount of javascript code needed to create a modal popup in jQuery (e.g.) is less than with Ajax Control Toolkit MPE. For example, I like the jQuery plugin SimpleModal. The code to turn a div into a modal popup using SimpleModal with default options is this:
<script type="text/javascript">
function showPopup() {
$('#<%=popupDivID.ClientID %>').modal({ appendTo: '#aspnetForm' });
}
</script>
Then you can just add that function to a click event, for example. And it's about a thousand times more flexible.
If you just don't like embedding javascript in your markup files, you can still technically control this all from codebehind by using ScriptManager to register the scripts and add events to controls.