Adata Nobility Series NH92 2.5 inch 500 GB External Hard Disk  (Blue)

If you're willing to pay a little extra for a slimmer form and not looking for USB 3.0 then this is the perfect portable hard drive. At 12mm thick for 1TB its slimmer than anything the standard brands (WD, Seagate, iOmega,...) offer. Works perfectly with a MacBook Air. Adata is a relatively younger company based in Taiwan but has good quality products. Drive has strong aluminium case and comes with a nice leather pouch as well.