If you aren't afraid of putting modified firmware on your camera, Magic Lantern is a nice (free) alternative. The intervalometer functionality is built into the firmware, so there's no need for any external device. I've used it and it works pretty well.
On a side note, the firmware also unlocks a whole bunch of features and options that you wouldn't normally have access to using Canon's official firmware. I'm pretty sure it voids the manufacturer warranty though.
Here's a link to the wikia if you're interested:
http://magiclantern.wikia.com/wiki/550D