I bought a hard drive enclosure off of eBay. Support USB 2.0 and FireWire. Bought a 40GB hard drive for it and I've been using it since to backup my important data.
They're much cheaper than retail external hard drives, and they are virtually the same!
Only noticeable differences are that retail external hard drives usually come accompanied with some kind of backup and recovery software to help you backup data periodically. But you can get these kinds of programs for Hard Drives anyway.
Another difference is that they tend to have some kind of 'One-Touch' backup feature, like Maxtor has. But this is for lazy people
A thing you need to look out for is that if you buy your own external hard drive enclosure, try and buy a good quality one. For example, the one I bought from eBay doesn't dissipate heat very well. Extremely hot when you touch it, doesn't have any fans, added heatsinks or air flow holes of some sort. It's like a tomb!
Also metal ones might be better, than plastic, because they are much more thermally conductive.