You could buy a cheap PC and run Linux or a BSD or Solaris or something on it using a volume manager to make a RAID filesystem and share it using Samba or NFS.

BTW, NAS==Network Attached Storage, which just means a remote filesystem accessible over your normal data network, a la SMB or NFS or AppleShares (whatever they're called) or whatever. SAN==Storage Area Network, which is a network devoted solely to storage media, usually connected to multiple host machines. Definitely not a consumer project.