I've just installed MozyHome Free on the family's laptops. 2Gb of free online storage per PC. Mozy runs in the background saving a copy of your data files online so if your PC suffers a catastrophic failure you can restore your data from Mozy's servers.
2Gb of storage space per PC is ample for us as long as music is excluded from the backup. If you want to backup a lot of music or photos you might find 2Gb is not enough.
I'm still using Carbonite to backup my own PC as I need unlimited storage space. I'm very happy with Carbonite but they don't offer a free option for small storage requirements.
If you're looking for unlimited online storage it's worth checking out both Carbonite and Mozy.
With free online storage available from a number of providers there's no reason for anyone with broadband not to use it!
With thanks to Derrick for mentioning Mozy to me.
