RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
Google Open Source - All of Googles open source projects under a single umbrella
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.
Opensource Builders - Find open-source alternatives to commercial apps
rubular - A ruby based regular expression editor
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.