This is not a tutorial on how to setup autotest on your machine. People have already done that plenty of times, a good one is here.
I'm just writing to say what a big difference it's made for me. My first job was at a "test-everything" development shop. I really agreed with the notion of having solid tests surrounding all possible code, and running them before every commit/deployment. The problem for me arose when I moved to start my own business. Without all that peer pressure (I'm working as the only development talent currently), it's easy to fall off the wagon. Especially when using tools that don't exactly integrate testing into your workflow. Typically I'd write tests around all new code, and run them before committing, but if I was making a quick change just to format something better or to fix a bug, I was often hurried enough to not only write no new tests, but to not run any of my current tests before committing just to get the damn thing out the door.
Autotest silently runs in the background, running your tests anytime you change a file. Not only that, it can be configured to use Growl to notify you every time a test breaks. Now I don't even have to think about it. Every time I press command-S, my tests get run and I know that at the least I haven't broken anything that's currently covered.
Of course the limitation is that you must write tests in the first place. Having your tests run all the time without any real coverage doesn't save you much. For me, though, just knowing that my tests are being run consistently gives me more motivation to write more of them, more often. If you're a rails developer, just try it. It's really not too much of a time commitment to set up, and if you're like me you'll be suddenly one giant step closer to being the unit-testing-guru that you always wished you were.