One for the coders: I discovered CodeRunner by Nikolai Krill a few months back, and it’s proven to be very, very useful in the kind of work that I do. It allows you to test blocks of code and see the results quickly and easily. It has syntax highlighting and autocompletion, as well as the ability to handle a wide variety of languages.
CodeRunner can also handle code that requires input at runtime, providing a field for passing parameters and input to your functions as needed. In short, it does just about everything I could ask a code-testing application to do.
There are free apps for testing code, but very few that can handle multiple languages and none that I know of which can handle everything that CodeRunner does. If you code in more than one language, CodeRunner is worth every bit of its $4.99 price. Go get it.
Nikolai also has a regular expression testing app in the Mac App Store called Patterns. I highly recommend it as well. The only issue I have with Patterns is that it doesn’t handle highlighting sub-matches, but for 90% of what I need it to do, it’s excellent.