Nikhil Kothari a software architect on Microsoft's .NET Developer Platform group has created a free tool called Script# that allows you write C# code that will be compiled into JavaScript. Using Script# you can take advantage of all your existing .NET tools and the power of Visual Studio. According to nikhil's own site Script# is used extensively within Microsoft when building Ajax experiences in Windows Live and Office along with external companies such as Facebook.

The Script# web site is http://projects.nikhilk.net/ScriptSharp, you can download Script# for Visual Studio 2005 or 2008. Along with the plug-ins for Visual Studio there is a very good PDF Readme that contains lots of examples of using Script#.

Nikhi also has a roadmap for Script# two notable additions are planned for 2009 Unit testing support and jQuery support. This is something I'm playing with and I think is worth checking out.