With the new, much more guitar-like Rock Band 3 controllers on the market, it's actually possible to configure them to create MIDI music in GarageBand on a Mac. Mac magazine Mac|Life shows us how.
The Rock Band games continue to innovate, the newest controllers coming with six actual strings for strumming and loads of buttons on the neck, each corresponding to a string/fret combination so you can actually learn to "play" guitar. With just the new Rock Band controller, a MIDI-to-USB adaptor, and GarageBand on your Mac, you can set up the guitar to record MIDI music that corresponds to what you're playing. It takes a bit of tweaking inside GarageBand, and a hefy sum of money for the new controller (which costs as much as an actual guitar—but if you're a big Rock Band fan, you may have it already), but the end result is actually pretty cool. Hit the link to see how it works.