CrashPlan is a commercial backup solution. It has a significant free component, but is closed source.
CrashPlan ( http://www.crashplan.com ) is a backup system that provides a richly featured backup client and multiple destinations for home and SOHO users. It's loosely bundled with a cloud storage service for the backup images, which is apparently the company's business model.
The freely available software allows destinations on network volumes, local folders and friends' computers in addition to the vendor's cloud storage facility. This functionality is unimpaired by the end of the trial period. Linux, OS X and Windows are all supported.
A network of friends in a given region could form a co-op, and, using the software, obtain for themselves fully redundant on-site/off-site backups for the cost of the storage alone.