Ajax.NET is a great little library of AJAX functionality that Michael Schwarz wrote in his free time.  It looks like the developement of this library has been taken over by a commercial entity, but appers to still be open source.

The Ajax.NET site not only provides some great examples of how to use Ajax.NET, but also demonstrates what AJAX is capable of accomplishing.  If your new to AJAX, and you want to see some example uses of AJAX, I highly recommend checking out this site.  Even if you are not going to be using the Ajax.NET library or ASP.NET.