I find the regular expression toolkit tool in Komodo 4.1 pretty handy. Trouble is you have to dish out $300 for Komodo to use it. I'd love the same functionality in a stand alone tool... any suggestions?
If using the JavaScript regular expression engine is acceptable, there's a nice one written by Erik Arvidsson. It doesn't have the "Replace" features that Komodo IDE's Rx Toolkit has, but it's helpful for testing matches.
He wrote for use with Komodo Edit - I guess he didn't want to shell out for the IDE either. ;)
Disclosure: Sridhar and I work for ActiveState, so we tend to push Komodo IDE. If $295 is a barrier for you, just tell our sales folks that you're a friend of Sridhar from stackoverflow and they might be able to swing a deal for you. We'll take it out of his pay.
Update: More shilling if that's OK. There's a sale on till the end of October. $50 off!