One of the many cool features of GarageBand is its ability to play back loops at any tempo without changing the pitch. But an even cooler feature is that starting with version 2.0, GarageBand allows users to create their own loops, which appear in the loop library right alongside all the factory loops. Here's how you do this:

First, trim the track down to the portion you want to use as a loop. You do this by positioning the playhead at the beginning of the desired audio and then choose Edit > Split (or Command-T). Also do this at the end of your segment. Make sure the loop you just made is selected, then choose Edit > Add to Loop Library. You'll need to give the loop a distinctive name, then you'll select Loop or One-Shot. (One-Shots are things like orchestral hits or cymbal crashes, that don't repeat.) Finally, choose the scale, genre, instrument, and mood descriptions. The ability to do this in what is essentially a free application that ships with your new Mac (within the iLife suite) is pretty amazing (and, we might add, highly addictive!).