This cool little tool from Ultrapico saved me a ton of time while attempting to whack together a fairly complex regex today. Though it can be a bit clunky at times, in general Expresso is extremely useful for visualizing how your regular expressions are stepping through test data. The layout is simple and intuitive. And best of all, it’s free.
Also played around with Jan Goyvaerts’ RegexBuddy, which is definitely the more slick of the two (real-time search while typing) and highly recommended by flagrant regex badass at large Steve Levithan. Nevertheless I found it easier to see what was going on in Expresso and, as I may have mentioned above… free.