So I drudged through a bunch of different Ajax libraries freely available on the web. I'm a bit reluctant to build any apps w/atlas naturally since it's not out yet. (On a side note, I couldn't really find any useful Atlas articles on the web - anyone got any?) Initially, I played with Michael Schwarz's (I needed to build some dependant drop down lists) However, this library was a bit lacking in one important area. ViewState. I couldn't access the ElementID.SelectedValue property as you normally would with a .NET control. The search continued...and ended with Anthem.Net
This library does everything that Michael's library does, while preserving ViewState. Also seems to have a good amount of sourceforge.net directly