I love bus-powered devices, especially external hard drives, but they tend to have fairly small capacities. I do like, those, these new Other World Computing Mercury On-The-Go portable drives.
They’re compact, bus-powered, work with Firewire 400 or USB 2, and even look cool. What’s really great is they’re high-capacity and fairly affordable, topping out with a 200GB drive for $189.99. If you have a laptop with limited capacity and need to expand, these are a great way to go.