The Mercury-On-The-Go from OWC is a somewhat tempting three-way connectible 500GB portable HDD. Component-wise there are no surprises, under the hood is a 2.5" 5400 RPM Hitachi Travelstar 5K500 with an 8MB cache. With Firewire 400/800 and USB 2.0 you get three-way connectivity and bus power.
I don't think that there is quite enough cool looking circuitry in a portable HDD to warrant a clear case, but the design works. At 3.5" (W) x 5.5" (D) x 1" (H) and at 2.5 lbs it is as light as can be expected. The kicker for the Mercury is the price; it is $359.99, which means that you are going to want to go on the road with your equipment a lot in order to justify the expense. If you work from home a 3.5" will give you a lot more bang for your buck. [OWC via Slippery Brick]