lacie is great.

www.lacie.com/products/range.htm

I just purchased a Seagate 7200.10 320GB drive for cheap and put it into an external box and now I've got a super fast drive for virtually nothing.

My recommendation for an external hard drive is Seagate. They have great specs and a wide selection at good prices with a 5-year warranty leaves the competition in the dust. Most HD manufacturers don't offer 5-year warranties - Seagate does. I have 6 Seagates ranging from 250GB to 500GB. No problems to report either. www.seagate.com/products/personal
Look for an external hard drive with a high RPM like 7200 and a good buffer like 16MB though 8MB is fine. Something else to keep in mind if you're purchasing a complete HD that will attach to your computer or getting a separate drive and placing it into a separate enclosure (tends to cost less and gives you more customizing options) is a Firewire 400 port. The standard USB2 connection is fine, but the Firewire 400 will add extra speed to accessing your iTunes Library off the external drive. A Firewire 800 port is even better. More recent Macs have both 400 and 800 ports.
Good luck!