In the November issue of The Oldie I wrote about how good I thought the online back up service offered by Mozy is - it's free, provided that you don't need more than 2 Gigabytes (and that's plenty unless you are storing masses of pictures and music). Also, once set up it is automatic, and painless. You are hardly aware of it, don't have to remember to back up.
I moved to Mozy from a similar service provided by BT mainly because I was paying £6.99 per month to BT for half the storage space I get at Mozy. I now suspect that I am not the only one, because BT have just announced a massive price drop - they will now give you 20 Gigabytes of storage for £4.99 per month, and are also offering a free 2 Gigabyte service.
Don't be tempted by them. The free service does not include the automatic back up system (which is what makes it all so easy to use) and anyway Mozy will give you 30 Gigabytes of storage for $4.95 per month.
At the moment, Mozy is certainly the way to go. You can see for yourself by clicking here.
PS - It's not (yet) available to Mac users, but they are working on it.