Ajax.NET - I'm impressed!
Historical article · Originally published June 2, 2005; technical details may be outdated.
I am totally impressed with Ajax.NET. And I'm not even a web programmer at heart :)
If you're doing ASP.NET development and find you need to do a lot of rich UI with various asynchronous things going on - this should help you out a lot!
You'll find lots of cool examples on the page I mentioned and here a re a couple more:
Brady Gaster : Ajax + Flickr