Anthem is 1.0.0! It's free, it's AJAXy and it's available now.
"Anthem.NET is a free, cross-browser AJAX toolkit for the ASP.NET development environment that works with both ASP.NET 1.1 and 2.0."
There are lots of great AJAX libraries out there, but I'm partial to Anthem. His stuff is ViewState aware which helps you in a number of ways and extends the server-side metaphors in a very natural way.
As an aside, interestingly, Community Server uses a private branch of Anthem. ScottWater uses both Anthem and ComponentArt's stuff, drawing a distinction between using Anthem when raw data is sent to the client versus using CA's when the UI is updated.
Most of all, I like that Anthem is fully supported in .NET 1.1. Not everyone can upgrade to .NET 2.0 tomorrow when they've got paying clients today, myself included. Do check out the source, it's pretty clever and elegant (remembering that AJAX itself is a huge hack. ;) )