Amazon's S3 is an online storage solution; you pay for only what you use ($0.15/GB/month, plus some transfer costs). I wrote a simple step-by-step guide to setting you a Mac to sync with Amazon S3; here's the executive summary version:
DiskWarrior is the most highly decorated Mac disk repair utility ever. It repairs disks that no other program can repair. New hardware monitoring helps protect your data from drive malfunctions.