FreeNAS is a free NAS (Network-Attached Storage) server, supporting: CIFS (samba), FTP, NFS, AFP, RSYNC, iSCSI protocols, S.M.A.R.T., local user authentication, Software RAID (0,1,5) with a Full WEB configuration interface. FreeNAS takes less than 32MB once installed on Compact Flash, hard drive or USB key.
This comes as a sugestion from sanbarrow.com:
Sounds a really great option for those who have two or three old computers getting dust and want to test out a network cluster for storing files..
Here's more info from wikipedia: