Most of the online regular expression testers I’ve used just aren’t that useful, but Rubular is the exception. It uses AJAX appropriately, has a clean aesthetic, and the quick reference at the bottom of the page is so useful it hurts. Plus, it has great features such as match extraction and the ability to create permalinks to your results.

Rubular is only for Ruby regexes. I wish somebody would make one for JavaScript. Jubular, anyone?