CrashPlan is a cross-platform, off-site backup solution. Basically, their software allows you to schedule remote backups to a separate server. What makes Crashplan different, however, is that the offsite server is a machine you choose. So for instance, if you have two computers you can set one to back up to the other. But even better, if you have access to an internet connected computer at another location (e.g. at work or even a friend's computer), you can install CrashPlan on that computer and then use it as a backup option (over the internet). Since the software runs on Windows and Linux as well as OS X, the off-site machine can be running any of these OSes.
CrashPlan comes in two editions: the regular one allows backup once a day for $20 and the Pro version allows "real-time backup + unlimited versioning" for $60. Unfortunately, from what I can determine from their FAQ, it appears that one license only allows you to backup on one direction. So if you want to backup two computers to each other, you must buy two licenses. Nonetheless, this looks like an interesting option, particularly given its cross-platform functionality.
[Edit: Corrected the pricing]