They make a bunch of cheap, single hard drive NAS's now. Not bad if you don't care about RAID or support for every type of media server and network protocol.
I'll also say that if you're in the iTunes world, the only solution that truly supports all your iDevices well is a Mac running iTunes 24/7. In my case, I ditched the multi-disk NAS (Infrant ReadyNAS) and just leave my MacPro on all the time.