Ajax.NET - A free library for the Microsoft .NET Framework
MonoRail (former Castle on Rails) is an attempt to provide a port of Action Pack (http://ap.rubyonrails.org) for .Net. The Action Pack way of development is extremelly productive, very intuitive and easily testable.
DbCombo is an ASP.NET server component for picking from long lists of data. It uses client scripting to 'auto-complete' a text box while you type - just like a combo box.