I like to use an external LaCie hard drive. I have a Firewire 400 that I used to use with my iBook before it passed away. I currently have a LaCie 250GB external that I use for my Dell laptop back-ups via Retrospect software. IF you had to use a third party online back-up, Digital Forest in WA is good for Mac. Not sure for PC. But definitely use software compression and good encryption of your data before transmitting over the open on the Internet, as it is all in clear text if the sessions are not protected by SSL.

Last comment: Western Digital has some cool 1TB external drives too. Ideally, I'd suggest a combination of 2-3 means and regular rotation of the media being used. Of course, that can get expensive, but if the data is mission critical, I'd spend the extra $$$.

Eric