There are plenty of backup services available that offer small amounts of storage space at no cost, but very few offer unlimited storage. The first two that come to my mind is Allmydata which charges $4.99 a month for unlimited storage, and Mozy Home which charges $4.95 per month. So for about $60 per year you can walk away with all of your important data safely backed up.
There’s also another service called Carbonite that recently launched a much acclaimed version 3. Their unlimited service costs about $50 per year, and with that you get a backup solution that is heavily integrated into Windows. With Carbonite installed your files and folders will have some added options to the right-click menu to control the backup process, and the central backup center (pictured above) keeps you up-to-date with how the backups are going.
I haven’t tried Carbonite out myself, but they do offer a 15-day trial so that you can see whether you would like the service before you get knee-deep in it. It’s also supposed to be available in retail stores such as Staples, Best Buy, and CompUSA.
Note: Right now it is only for Windows, but a Mac version is scheduled for release in October.