RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
regular expressions 101 - Extensive regex tester and debugger with highlighting for PHP, PCRE, Python and JavaScript.
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.
.NET Regular Expression Designer - Learn, develop and test Regular Expressions for the .NET platform.
RegexMagic - Use RegexMagic to generate complete regular expressions to your specifications by using RegexMagic's flexible patterns, instead of the cryptic regex syntax.
Debuggex - Debuggex is an intuitive visual regular expression debugger, tester, and helper.