I've been pleased with rdiff-backup so far. I use it to back up data from my server at home to my remote shell account where my shell provider does the quote-unquote "real" backup.

rdiff-backup is nice because uses librsync to intelligently handle file differences, it does incremental backups, it lets you easily specify which directories to back up, paths/extensions to exclude from the backup, etc.

I know folks here are steering you towards tape, and with good reason. But if cost is prohibitive and you need to just backup to another system (whether it's local to your office or hosted elsewhere) I think rdiff-backup is a good choice, and it addresses some of the limitations of conventional HD backups that matthew_k mentions above.