My friend Panos wrote today about Nikhil Kothari's Facebook.Net framework (hosted at Codeplex) as well as Steve Trefethen's starter kit as various tools you could use to develop Facebook applications using .Net.
I'd like to add to those The Facebook developer toolkit that evolved from the Microsoft - Facebook partnership. Although this download comes from the Microsoft Visual Studio Express Editions product line (as you may notice) nothing stops you from using it with Visual Studio.
Furthermore (if you're willing to play with cutting edge technology ;-) you could give Popfly's Facebook blocks (which are products of the same partnership) a try and develop rich Silverlight web applications...