Other World Computing makes my favorite 2.5-inch hard drive enclosures, bar none. Their Mercury On-The-Go cases are killer because they're made of clear Lucite so that you can instantly tell which drive is inside just by looking at them. I know, I know, stickies work too, but stickies come off and if you move your drives around a lot it can be a pain to find the correct drive.
In addition to being clear (with piercing blue and red LEDs) they're wicked fast and come in several flavors depending on what interfaces you need. Personally, I opted for the trio of FireWire 400, 800, and USB 2.0 connections, because that allows me to use the drive with my MacBook Air (which is only USB) and on my MacBook Pro at full FireWire 800 speeds. They also offer a USB 2.0 + eSATA configuration (do any MacBook owners out there actually use eSATA?)
The enclosure itself is build around OWC's custom bridge board and the Oxford924 chipset. The highest capacity portable, bus-powered storage drive to date is the OWC Mercury On-The-Go 500GB, 5400RPM portable drive. The Mercury five hundy comes with a Hitachi Travelstar 5K500 mechanism inside for US$360 and makes a perfect companion drive for MacBook Air owners suffering with a tiny 80GB hard drive. Hello huge Aperture and iTunes libraries!
Don't get any ideas though. The Hitachi 5K500 mechanism is 12.5mm tall so you can't tuck one into the MacBook or MacBook Pro 15-inch which only have room for a 9.5mm tall drive. MacBook Pro 17-inch owners can fit the taller 12.5mm drives into their machine.
OWC Mercury On-The-Go drive ship with a carrying case; cables for all interfaces specific on-the-go model supports; Prosoft DataRescue III for OS X; and NovaStor NovaBackup for Windows. All drives are covered by a one-year warranty and are worth every penny.