I'm playing around with some ideas for a new testing tool for RegexLib.com:

    http://regexlib.com/BetaTester/TestPage.aspx

This tool will allow you to test patterns using clientside (VBScript or Javascript) or, serverside (.NET) engines.  When using the .NET engine you can also choose to display results in a Grid (group view) or a Tree (captures view).

Feedback
I should have it tested and integrated in with the rest of the site within a week (hopefully).  If you have any feedback, or, requests for additional features, please either mail me via my contact form or just leave them as a comment below.

Thanks to Moshe Salomon for the client-side bits :-)

Known bugs:
.NET Named groups throw a syntax error when using the angle bracket version, use the single quote version for the time being: i.e.: use (?'nameOfGroup'...) instead of (?<nameOfGroup>...)