I'm studying for 70-646, the 2008 Server Admin test. Read about Storage Area Networks (which offer fast iSCSI access behind file servers) and Network Attached Storage (which basically is a file server and offers access directly to workstations).
A free SAN OS, OpenFiler, looks really cool. I want to try it out. Started reading a discussion about it here. OpenFiler is said to be fast and comprehensive - it's also the only free SAN platform that I'm aware of.
A free NAS OS, FreeNAS, looks quite slick and very current/active. It does offer iSCSI. It also supports the ZFS filesystem, which is targeted at very, very large storage arrays where silent errors have a statistically higher probability of occurring. ZFS offers online disk repair (unlike CHKDSK), checksums for all disk writes, a certain amount of roll-back via copy-on-write, and tremendous volume sizes. I decided this is not relevant to my life right now, but FreeNAS and OpenFiler both are. Sometime it would be worth coming back and reading this short overview of building a small SAN.