Windows: If you've considered using remote backup to keep your data safe from disaster but you're wary of putting your backups on a third party server, Cucku Backup is for you.
Cucku Backup is a free application which allows you to automate local and remote backups. You can set up Cucku backup to backup your data to a local drive, external drives, and network storage. The feature that sets Cucku Backup apart from the host of applications that allow you to schedule local backups is the ability to send encrypted backups to be stored on your friends' computers. Using the Skype network as a backbone—you'll need to sign up for a free account if you don't have one already—Cucku Backup encrypts your files with 256-bit AES encryption before shuttling them off to the friends you are sharing storage with.
Cucku Backup is currently best suited for backing up critical files due to the 4GB per file size limitation and, of course, the limits of your friends' willingness to donate their entire hard drive to backup up your collection of Frasier DVDs. For another friend-to-friend backup application make sure to check out Zoogmo, and for backup solutions both local and remote check out the Hive Five: Five Best Windows Backup Tools. Cucku Backup is freeware, Windows only.