FreeNAS

[Dos_Box]

As featured in the latest PC Pro.
www.freenas.org
Not quite finished yet as LDAP authentification is still being worked on but it is a tiny install. They also allow you to download a preconfigured VMWare virtual PC which is great for testing. It runs off FreeBSD and is a wonderful way of ressurecting a collection of old hard drives into some free network storage with almost zero effort.

[webman]

Yep, tried it at home a few times, it's really really good for a quick and dirty NAS. Highly recommended

[plexer]

Looks good.

www.openfiler.org is a more heavyweight solution as well.

Ben