{
Steven mentions Nregex in a list of regular expression testers online. Nregex is still up and still useful especially if you need to work with .NET's implementation of regular expressions. A coworker of mine just used it to build a parser for Sql Reporting Services RDL files. Steven's own RegexPal is a fairly intense implementation of regular expressions in javascript, complete with syntax highlighting.
Yes, I think it's easily the best online .NET regex tester. You are planning to keep it up, right? I'm hoping to recommend it for testing .NET regexes in an upcoming book, but if you're thinking of taking it down, I'd appreciate a heads up so that I can look elsewhere. (I only mention this because of the "still up" notice in your post.)
Once again, great work!