Cross-browser DHTML widgets toolkit

ActiveUI is really an awesome piece of job!

It's not .Net but I am sure it could be adapted as a user control.

It surely not for the newbies but it has a lot of tempting things like the fact it's cross browser.

Download the source on sourceForge

ActiveUI is a modern cross-browser Web GUI toolkit. Provides full set of standard widgets (grid, tree, tabs, menu, etc.) with fast and convenient data-binding interface. Built on JavaScript/XML/DHTML/CSS, integrates with JSP, ASP.NET and other back-ends.