It doesn't need to be 7200 rpm, just small and ideally super robust.
Exact same external as GamaFu's post above - it is a great little drive indeed. I have it hidden inside my desk and it works flawlessly as my "out of sight out of mind" time machine HDD.
If you want a small hard drive that works in Windows aswell,
get a Lacie hard drive. small and portable
I've got a Western Digital MyBook Home, connected by Firewire, and it works great. I think it's pretty nice looking, but it's no where near as small as the Passports.
It doesn't need to be 7200 rpm, just small and ideally super robust.