A NAS device would be perfect for you (Network Attached Storage). See FreeNAS or OpenFiler. They support multiple protocols, i.e. CIFS, FTP, NFS and AFP, plus iSCSI which, while complicated (really a business oriented SAN technology) would allow you to mount volumes on the NAS box as drives on other machines. Maybe a bit much for home enthusiasts but they're both quite popular in the enterprise...