The Express enclosure is just slightly larger than an iPhone at 3.07" x 5.12" x .55" (78mm x 130mm x 14mm), and weighs just 2 ounces (56g) empty. Just toss in your old 2.5" SATA drive and you have a bus-powered USB 2.0 drive that easily fits in your pocket. You can also shop around for new bare SATA drives; a 500GB, 5400 RPM drive can be had for as low as US$97 (I'm sure you can find them for less!), making for a low-cost and portable backup solution.
There are other similar USB 2.0 enclosures out there; MacAlly's PHR-250A (US$19) and StarTech SAT2510U2 Infosafe (US$17.96) are about the same size and weight as the Express, while the Vantec NexStar SX NST-285S2-BK (US$22.02) and Sabrent EC-UST25 (US$14.97) are larger and heavier.
What's your favorite use for disk drives that you've pulled out of your Macs? Leave a comment below.