RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
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.
rubular - A ruby based regular expression editor
RegexBuddy - RegexBuddy is your perfect companion for working with regular expressions.
RegexPlanet Ruby - RegexPlanet offers a free-to-use Regular Expression Test Page to help you check RegEx in Ruby free-of-cost.