RegexPal (easily the most del.icio.used regex tester 
) is now released under the Creative Commons Attribution-Share Alike 3.0 License GNU LGPL.
There are certainly many more features that can be added to the app and things that can be improved, so if you are interested in helping out or creating your own version, you are welcome to do so. If there is interest I'll create a Google Code project, but for now there is a package you can download which includes all files for the regexpal.com website. Two of the files in the package (xregexp.js and helpers.js) are dual-licensed under the MIT License.
If you're only interested in the JavaScript, you can see the three source files here:
For regex aficionados particularly, there is some stuff here you might find interesting, including the latest, as-yet-unreleased version of my XRegExp library, and the regex syntax parser used for RegexPal's syntax highlighting (which includes lots of details on the minutiae of regex syntax and cross-browser regex handling).