CrashPlan is an interesting free backup tool that's a little different to most of the competition.
You don't have to spend a great amount of time figuring out how to set it up, for instance. By default the program will automatically back up everything in your User folder (Documents, Pictures, Videos and more). And once you've specified a backup destination, CrashPlan is always running in the background, backing up new or changed files. (Don't worry about it interfering with other software, though: if it detects you're working at the PC then it'll use the minimum of system resources.)
And your choice of background destinations is interesting, too. As well as supporting external drives, the program can back up to another of your computers, or even a friends PC (over the internet), just as long as they also have CrashPlan installed. So whether your friend is in the next town, or half way around the world, you'll be able to back up important files to their system at no charge at all (don't worry, they're encrypted for safety).
If you'd prefer a more conventional online backup solution, though, CrashPlan has other plans which may appeal. So CrashPlan+ offers 10GB from $24.99 a year; CrashPlan+ Unlimited has no storage restrictions and is priced from $49 a year; and CrashPlan+ Family Unlimited takes care of your backup needs for up to 10 computers, and is priced from an annual $119.99. Find out more on the CrashPlan site.