RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
rubular - A ruby based regular expression editor
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.
regular expressions 101 - Extensive regex tester and debugger with highlighting for PHP, PCRE, Python and JavaScript.
RegexPlanet Ruby - RegexPlanet offers a free-to-use Regular Expression Test Page to help you check RegEx in Ruby free-of-cost.
RegexMagic - Use RegexMagic to generate complete regular expressions to your specifications by using RegexMagic's flexible patterns, instead of the cryptic regex syntax.