Two years ago, I got a Seagate 250gb that's about the size of a deck of cards for about $50 from NewEgg. I've been very happy with it, though that's because my laptop doesn't have a firewire jack. If it did, I'd almost certainly get an external that could support something besides USB.
edit: between those two, I'm noticing the 250gb one says it's powered by USB, whereas the 500gb one doesn't say that. Needing a separate power outlet could be a deterrent depending on your workspace, and if you want to move the external often (e.g. from one computer to another) having to unplug it from USB and from the power outlet would be a big hassle.
This post was edited by bentherdonethat on Jan 25 2011 12:34am