Expresso - The award-winning Expresso editor is equally suitable as a teaching tool for the beginning user of regular expressions or as a full-featured development environment for the experienced programmer with an extensive knowledge of regular expressions.
RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
Regex Hero - Online Silverlight regular expression tester with instantaneous highlighting, C# and VB.NET code generation, code completion, regex analysis, benchmarking, and more.
RegexBuddy - RegexBuddy is your perfect companion for working with regular expressions.
Debuggex - Debuggex is an intuitive visual regular expression debugger, tester, and helper.