I've used two different services, with varying degrees of success. I'm sticking with the second one, ElephantDrive.
I started with Mozy - it was pretty good. Easy to set up, cheap, and simple. Problem was that I didn't realize that if I deleted something locally, it would get deleted from my account. Basically, Mozy is good for backup but not for anything else.
ElephantDrive basically does the same thing Mozy does for backup, except that it allows multiple devices and doesn't ever blow anything away unless you tell it to. It was a little more expensive for the home product, but actually way cheaper for the business level account. That's my reco.
One big caveat - I didn't try either of these on a Mac. Not sure what they'll tell you.
Here are the links:
http://www.elephantdrive.com
http://www.mozy.com
Artful D