Google Open Source - All of Googles open source projects under a single umbrella
rubular - A ruby based regular expression editor
Opensource Builders - Find open-source alternatives to commercial apps
RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
RegEx Generator - RegEx Generator is a simple-to-use application that comes with the brilliance of intuitive regex and is also helping you out to test the regex.