BackupPC

Second the recommendation of backuppc.  It's very easy to use, very easy to backup multiple
computers (including non-unix ones, though we don't have any), uses rsync, compresses its
storage, manages backup rotation (seriously, if you don't have periodic backups stretching
back at least a month, it's not a real backup solution), etc.

The key feature for us was that backuppc is smart enough to notice identical files on multiple
computers and only store them once; given that we have large, substantially overlapping music
collections on our various computers, this gives us effectively 2-3x more space on our backup
server.  As far as I could tell, it's the *only* free backup system that does this; dervish
had a feature that looked sort of similar, but it turned out to be a dumb hack.

Backuppc is also a quirky, overgrown mess of perl scripts; it's just been hacked on long
enough that it probably already has a quirk thrown in for whatever situation you're in.  It
would be so nice to have a backup solution that was designed properly from the start, though,
aiming directly at disk-to-disk backup, with a sane and smart storage backend, and a simple
API for retrieving files and otherwise managing the store.