RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
RegexBuddy - RegexBuddy is your perfect companion for working with 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.
Expressions - An app to play with regular expressions. Easily and nicely.
rubular - A ruby based regular expression editor
Patterns - Patterns is a simple yet powerful tool for working with regular expressions.