Mac OS X only: We've been fans of Time Machine ever since the system snapshot utility was introduced in Leopard, but if you'd love to take your Time Machine backups offsite, Time Warp might be up your alley.
Currently in beta, the application deliver Time Machine data of your choosing to Amazon's S3 storage, where you pay a relatively low price for transferring said data. Specifically:
$US0.15 per GB per month for storage $US0.10 per GB to backup data into S3 $US0.17 per GB to restore data from S3
Time Warp helps keep those costs as low as it can, compressing and filtering backups to just what you need. You can, for example, choose only to backup Time Machine data from your Home folder, specific folders within your home folder, or you can go hog-wild and aim for all of your data. You can keep track of your storage costs with Time Warp's configuration tool and scale back if Amazon costs go too high. Lastly, Time Warp encrypts all your backups to keep your data safe.
Granted, we'd still recommend using something like previously mentioned Mozy for your serious backup needs, but if offsite Time Machine backups are worth it for you, Time Warp seems like a good solution. The application is currently free while in beta, will cost $US25 after that. If you'd simply like the option to do your Time Machine backups over a local network, check out previously mentioned iTimeMachine.