I really like this Kickstarter project. It turns a MicroSD card into an additional drive in your MacBook Air or MacBook Pro. It fits into the SD card reader of those computers, but instead of sticking out like normal SD cards, it is flush to the side of the computer.
While my computers are backed up both locally using Time Machine and to the cloud using CrashPlan (not to mention DropBox for documents and text and Evernote for PDFs to the cloud), backing up while on the road is not as easy. Since MicoSD cards now go up to 64 GB, this could serve as a decent backup. While this will not save you if your computer is stolen, since the card would be in the stolen computer, it adds one more layer of backup.
I backed it on Kickstarter.