I have a Western Digital 80GB external that I believe cost me about $60 (this was last summer though, so you might be able to find it cheaper now). It's pretty small (about the size of a passport and maybe an inch thick) and is USB-powered, which is awesome. I've had a few problems getting it to work with my iBook, but I'm not sure if it's the hard drive or my computer. It's worked fine on my PC since last July with no signs of stopping.