Setting up automatic red / green notifications for continuous testing and TDD?

*Note: this was originally posted under stackoverflow, and someone sensibly downvoted it since it's really OS X specific. So I'm reposting it over here, since it's been generally helpful.* I like doing Test Driven Development (TDD), and so I would like a way for my test suite to run whenever I make a change to a source file or test file. In addition, I'd like the results of the test to display in an ephemeral "Growl" style window in the corner of my screen, showing a green icon if all tests passed and a red icon if one or more tests fail. I know there is such a mechanism for node.js in mocha, and for ruby environments there's guard-rspec. But how do I create one for my [fill in the blank] testing environment under OS X?