Dropbox is the easiest way to store, sync, and, share files online. There's no complicated interface to learn. Dropbox works seamlessly with your operating system and automatically makes sure your files are up-to-date. Available for Windows, Mac, and Linux.
rsync hfsmode is a patch for rsync to enable recognition of Mac OS X HFS resource forks and Finder metadata, and to copy them to a remote filesystem. The destination system can be any OS and filesystem that supports rsync, so you can use rsync to archive Mac OS X files to servers running Linux, Solaris, et cetera.
With the new version of OS X (Leopard) Apple has included some great functionality in Time Machine. Your Mac will automatically backup to an external drive every hour. It includes the ability to recover deleted files in a timeline.
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: