Hey Sniper, are you looking for software to make a Linux server into a NAS or are you looking for a NAS to connect to a Linux server?
If the former, I really don't know of too many programs that are turn-key. You can always do it by hand and set up a NAS (either for Linux/Unix or Windows). If you are looking for a NAS that uses Linux, then there are a few out there.
If the latter, then let me say that almost any NAS will work for Linux. Most NAS that I've seen have NFS/NIS and SAMBA capabilites. Then again, I'm no NAS expert.
There is one called
OpenNAS which advertizes a lot. 1 Tera Byte for around $23.000.
I think
Einux also has some small NAS servers.
VA Linux is also in the Linux NAS market.
cl