RegExhibit
If you've ever been stuck with the problem of trying to build anything but a simple regular expression you know how painful it can be getting it to match just what you want.
When developing for .NET on Windows I was introduced to a brilliant free tool called Rad Software Regex Designer that gave you the ability to provide an example of the text you wanted to match and an area to slowly build up your regular expression while getting instant feedback on what it was doing. It even has dialogs to add specific regular expression commands in case your proficiency with regular expressions isn't high or you just forgot how to create a non matching group. After moving to OSX for work I went looking for a similar tool for the Mac. And after a while I found it.
RegExhibit is a GUI tool of OSX that uses the Perl regular expression library to help you build regular expressions. This should be fine for any other languages that use a PCRE library but make sure you check before deploying. The core part of the program are two text areas. you place an example of the text you want to match into the lower area and build up your regular expression in the top are. There are even tabs for doing matches and splits but you'll likely find yourself in the match tab for most of time. However it doesn't offer the same built in dialogs like the Rad Software Regex Designer so make sure you've got a regular expression reference handy.
This is a great tool that has saved my sanity several times already and I do recommend to anyone that has to play with regular expressions and is developing on the Mac.