NAS are very popular these days; for people who can not afford a professional NAS or a sub 1000€ system, the idea of recycling an old pc come quickly to mind. What is a little bit more difficult is to find the right hardware and software combination.
I higly recommend Promise SATA raid controller for their Linux native support, while for software I came across OpenFiler; an open source project which has professional functionnalities...

Openfiler is a powerful, intuitive browser-based network storage software distribution. Openfiler delivers file-based Network Attached Storage and block-based Storage Area Networking in a single framework. Openfiler sits atop of CentOS Linux (which is derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor). It is distributed as a stand-alone Linux distribution.

The entire software stack interfaces with third-party software that is all open source. File-based networking protocols supported by Openfiler include: NFS, SMB/CIFS, HTTP/WebDAV and FTP. Network directories supported by Openfiler include NIS, LDAP (with support for SMB/CIFS encrypted passwords), Active Directory (in native and mixed modes) and Hesiod. Authentication protocols include Kerberos 5. Openfiler includes support for volume-based partitioning, iSCSI (initiator), scheduled snapshots, resource quota, and a single unified interface for share management which makes allocating shares for various network file-system protocols a breeze.

Note that I am currently thinking on buying a ReadyNAS NV becuse of its size