Jeff Key's Snippet Compiler is simply amazing. It allows you to write short snippets of C# or VB.NET code and run them without the hassle of setting up a new VS.NET project. It is incredibly handy for running examples found in the MSDN docs.

Perhaps the most amazing thing about Snippet Compiler is how much development it's gone through in a short amount of time. The first version I started using was little more than a text box that compiled and ran the code. Now it imports VS.NET projects, exports snippets to HTML or RTF, and... would you believe it even has Intellisense??? I still can't.