I absolutely love the Clickfree Traveler. It's the size of a credit card, so it can easily fit in your wallet. Plug it in using the super-thin USB connector, and it automatically starts backup up everything on your computer—no clicking, installing, or approving required. Plus, it comes in 16GB, 32GB, and 64GB versions (16GB should be plenty for most people.)
The drive is a little tough to find online. You can get it at Clickfree for the list price ($80 for 16GB, $150 for 32GB, $250 for 64GB). Amazon is selling the 32GB version for $131.56. And you can get an even better deal at B&H: $76.50 (16GB), $126.95 (32GB), $229.95 (64GB).