These slim Apricorn Aegis 1.8-inch drives are both USB and Firewire compatible, making them suitable for Macs and PCs. They've got a shock mounting system and only weigh 3.5 ounces, so taking it on the road with for added laptop storage isn't out of the question—it's totally in the question.
For Mac users, there's also ShirtPocket's SuperDuper! backup software (that I recently ponied up for!) and Apricorn's encryption software.
Prices start at $159 for the 30GB version up to $239 for the 60GB version. Add $10 to both for firewire compatibility.