Many months ago I wrote a post on AJAX toolkits for ASP.NET. Things have moved on a bit since then. Mainly Microsoft ASP.NET Ajax was released.
OK I still have concerns that Microsofts AJAX solution is overweight. However thats what the cache is for ! Overall I think that I would use Microsofts AJAX solution because its likely to be ubqitous so you are plugging into a community of code and Microsofts continuing development and integration into Visual Studio.
Even better theres a free library of controls and extenders, with source code, that anyone can contribute to. You can find it here : http://www.asp.net/ajax/ajaxcontroltoolkit/samples.
This is a collobration between Microsoft and the community.
AJAX.NET Professional is still out there but development has now stopped.
Find it here : http://ajax.schwarz-interactive.de/CSharpSample/
Michael Schwarz did a great job on this in his free time and I enjoyed using this.
I havent used http://ajaxwidgets.com but have looked at a couple of demos and it looks really slick. Last time I looked it wasnt free but looks like they may have now changed the licencing model. Main issue its not Microsoft so not sure how it will survive long term. With this system you write what looks like serverside code, e.g. in an onclick event for a button, but its actually automatically actually run via AJAX call. I am sure there is more to it than that though ...
As I say my recommendation would currently be to use Microsoft ASP.NET Ajax for projects, though if you only want to use a little AJAX and want more control I still have a fondness for Ajax.Net