1. Install the ASP.NET Ajax 1.0 extensions here http://www.microsoft.com/downloads/details.aspx?FamilyID=ca9d90fa-e8c9-42e3-aa19-08e2c027f5d6&displaylang=en
2. Download this (AjaxControlToolkit.zip)
   and unpack it to a folder on your local computer.
3. Open Visual Studio, right click on Tools and create a new tab. Call it Ajax Controls.
   Right click the tab and pick Choose Items…
4. Browse to the directory where you unpacked the toolkit, browse to the Sample Web Site
   folder, then the bin directory. Choose the AjaxControlToolkit.dll and your list should
   be populated with the new controls.

If you want to see a sample of the controls in action, go here (http://www.asp.net/AJAX/AjaxControlToolkit/Samples/Default.aspx)

There has been a lot of buzz around SharePoint support for ASP.NET AJAX the last couple
of days, resulting in some nice posts, even from the Microsoft guys. If you are new
to the topic; some required reading:

• Integrating
  ASP.NET AJAX with SharePoint (by Mike Ammerlaan)
  Mike clearly described what you need to do to get the ASP.NET AJAX extensions working
  in a SharePoint site. First install the extensions (of course), then make some changes
  to the web.config of the SharePoint site.
• AjaxBasePart:
  Easy ASP.NET 2.0 AJAX Extensions 1.0 and Office SharePoint Server 2007 (by Eric
  Schoonover)
  Eric describes how you can make use of ASP.NET AJAX in a SharePoint web part, he posts
  code for his implementation of a base web part class that supports AJAX. The nice
  thing about this base web part class is that it dynamically adds the AJAX ScriptManager
  control to the class (required for the AJAX stuff).

Yesterday evening (very late ) I’ve created a version of the SmartPart that
makes use of the Eric’s technique, let me introduce you the SmartPart with
AJAX! So basically said: you can create a Web User Control (ASCX) with the
Visual Studio Designer, that uses the ASP.NET AJAX extensions and run that user control
as a SharePoint web part. At this point in time, it’s still a beta version, you can
get it from the GotDotNet workspace (releases
section). How do you get started? First of all you need to download and
install the ASP.NET AJAX Extensions. Next you need to extend your SharePoint site
with these extensions, check Mike’s
post for detailed instructions. You only need to change the web.config,
you don’t need to add the ScriptManager to the master page! Now you can install
the SmartPart (check the Installation guide, or this screencast).

SmartPart
for SharePoint – ASP.NET AJAX Support – Jan Tielens’ Bloggings