I'm starting to use .NET's RegEx class all over...in ways I never expected.  Here's the tools and sites I'm using lately:

• Expresso is a new free tool for working with regular expressions in .NET. [Larkware]
• Of course, RegexDesigner.NET from Chris Sells
• The Regular Expression builder inside of SharpDevelop (which is worth the download just to read the SharpDevelop code!)

And Library Resources:

• RegExLib.com has 213 Regular Expressions at last count (and is hosted by the kick-ass ORCSWeb)
• The Early Adopter Regular Expression Repository at 3Leaf
• The Regular Expression Workbench at GotDotNet.com
• Miloslav's Regular Expression Reference
• Dan Appleman's Regular Expressions with .NET book
• The Tao of Regular Expressions