If I wasn't a serious geek who built a storage cloud from scratch, I'd use the Western Digital World Edition. Out of the box, it's solid. Your favorite geek can customize it if needed because under the hood it's a Linux server.
That or use webserver old hardware you have laying around and install FreeNAS or similar. A comparison of open NAS solutions, all of which are more feature rich and customizable than are affordable NAS hardware:
http://www.linuxplanet.com/linuxplanet/reports/7153/1