The DIY NAS
NUS2000
Cirago
$80
This little box from Cirago isn't a hard drive. It's a smart USB hub that attaches to your home network – plug any USB drive into it, and that drive becomes network-accessible. It effectively turns as many as four USB drives into NAS drives. Each port can handle a 2TB drive, giving you up to 8TB of networked storage.
Settings are accessed through a browser-based control panel, and although the user experience is a touch hairy, more tech-savvy users shouldn't have problems getting it set up. It's not a Wi-Fi device, so it will need to sit relatively close to your router (an ethernet cable is included).
Via the browser, you can administer user accounts so certain people can only access certain drives, and you can set up remote access for your users. There's a print server, an iTunes server, and even a built-in BitTorrent client that can write directly to the attached drives. For only $80, this thing's a bargain, especially if you have a few hard drives lying around that you want to make available to every PC on your network.