Maxtor, LaCie, Seagate, Western Digital, etc., are all good brands.
Here's one:
http://www.ebuyer.com/UK/product/62833
Basically make sure it has a USB2 connection. If you have a Mac at all, get one with Firewire too (you could do this on a windows PC too if you wanted, but it's especially important for a mac).
So the only other choice is how much space you want -- the above is 250GB. This just depends what you want to do with it; the more the better, obviously, but these size drives are at a good price point right now.