The Philips 160Gb HDD is a very reasonably priced 160 Gb disk available from Tescos. It is powered over the USB cable, and also includes a Y-cable for low power USB ports. It is slightly larger than any other 2.5" drive I've seen as it is "shockproof" and has a fairly substantial aluminium and rubber casing.
I can't find it for less than £60 online, and the price seems low for a 160 Gb 2.5" drive.
The drive itself appears to be a Seagate Momentus SATA 160 Gb disk - type: ST916082 1A
This drive is 5400 rpm (as described on the box) - some websites incorrectly state it is only 4200 rpm.
Compatible with Windows and Mac OS, but you'll probably want to reformat it on a Mac as it is NTFS.