Mozy

Mozy is the most basic here and offers 2GB of storage for absolutely nothing, and a few larger capacities at competitive rates. There are various presets to tick, depending on what you want to back up: various office file types, My Favorites, media collections or there's a file-tree to create your own.

Scheduled backups can be set, but Mozy works best by monitoring modified files. However, it will only back them up differentially, when the system is idle. It uses 448-bit Blowfish encryption and you can use the Mozy key or enter your own as long as you keep it safe. Locked files can be backed up from NTFS-formatted disks, while older versions of files are kept for 30 days before they're deleted, and backups can be restored onto any PC via the website.

But few things in life are free: the user interface is painfully basic, plus the fact that it doesn't continuously back up files as they change makes Mozy less effective for vital work.