A fine example just came up on Instructables: How to Make a Cheap, Portable Magnetic Stirrer... with a computer fan!
Eventually, of course, it would be good to learn how to use the thing in greater depth. Perhaps you've followed that author's advice and ignored the third wire. That's cool. But some day in the future you'll be happy to learn what additional functionality that can provide. I believe it's a tachometer? Or maybe a thermostat on some fans? Obviously that day is still in my future, but I'm looking forward to it!