So here is a quick summary. If your web application is based on ...
- ASP.NET 3.5 - you need ASP.NET AJAX 3.5 and it comes built-in with VS 2008. If you need specialized controls from the AJAX Control Toolkit use Toolkit version 3.5. If you are specifically building on top of .NET Framework 3.5 SP1 using Visual Studio 2008 SP1, use Toolkit version 3.5.20820. (System.Web.Extensions, Version= 3.6.0.0 corresponds to ASP.NET 3.5 Extensions December CTP)
- ASP.NET 2.0 - you need ASP.NET AJAX 1.0 and you have to manually install it in VS 2008 or 2005 and make sure the web.config of that application correctly points to the 1.0 assembly. Toolkit version 1.0 will only work with ASP.NET AJAX 1.0.
- ASP.NET 1.1 - ASP.NET AJAX is not supported. You would have to choose a Framework like the free, cross-browser Anthem.NET which works works with both ASP.NET 1.1 and 2.0.
If you want to compare AJAX Frameworks for ASP.NET, this outdated article on the comparative features of various Frameworks can give you some pointers to base your research on.
Related links:
Free ASP.NET AJAX E-learning course
HOW TO implement AutoComplete with jQuery & ASP.NET
Is ASP.NET AJAX truly cross browser?