One of my coworkers in Visual Studio has created this really cool tool for creating and testing Regular Expressions.  I’ve just recently started using .NET Regular Expressions in my Editor testing, so take my word for it that this tool greatly helps with writing regular expressions.

Overview for the tool, as written on GotDotNet:
A tool for building and testing Regular Expressions. It allows you to manipulate the expression and your source text, and shows you a tree with all of the Matches, Groups, and Captures found in the text.

The tool can be found at
http://www.gotdotnet.com/Workspaces/Workspace.aspx?id=9e33c395-8275-4906-8a09-0bff41fdc1d6