I've recently bought a WD 1Tb drive which, so far, is fine. (Amazon - similar price to BOAC's comment). It sits there incrementally backing up a mac using Time Machine but, apart from random hard disc failure of the host machine, is just as vulnerable to theft/fire etc as the mac. I have other hard discs with, less frequent, backups of smaller selections of my data like photos etc which are cleverly hidden away in the sock drawer. They (photos) are the real treasures, along with a few documents, as almost everything else would be recoverable albeit time consuming.
I would like to investigate wireless hard disc options - like apple's time capsule - so that any backup disc could be hidden away and be less likely to be nicked - although maybe not immune to fire.
Otherwise I guess some type of online or over-a-network backup could be useful.
I'm off to copy my pictures to DVD.
Where's that paranoia smiley!