I'll second that. I've had a FreeNAS box up as a media storage server and backup for over a year and it's been great. 400MHz Pentium machine, RAID 1 on 2 400MB IDE drives, plus a 3rd 400MB IDE drive that is backup for my main workstation (which itself has RAID 1, so I'm not as worried about RAID on that backup).

My wife's Macbook gets backed up automatically every night using Superduper onto the NAS's CIFS share, and as does my Mac desktop. My web server and code repository (Linux) are backed up via streaming tar over ssh to the the NAS. Using syslog from the NAS share to the Linux box to monitor the health automatically, so I don't have to go through the web interface, just get an email if anything goes wrong (never has).

Only cost was the drives (about $400 if I recall correctly), and about a day to install and configure everything and set up the backups on all my machines. Getting the SSH right was actually the toughest, because it was hard to figure out where to store the authorized_keys files on the NAS where it would survive a reboot (sorry, don't recall how I finally solved that).