_I do not want to build my own_. I want a small box I can plug into the network and the power supply and have instantly running.
Ideally it should "sleep" when it's not being used so that it's not eating up too much power. And it should support both windows file sharing (SMB). UPNP is nice, but not required.
Edit:
It's for home use - just something to dump some files onto. Basically, Julie's hard drive has become full, and having somewhere to put the files would be good. I was torn between picking up a USB2 external HD or getting a cheap NAS-box. External HDs are £75 for 1TB, and I was wondering what the price premium was for something that could be stuck under the router rather than having to be physically plugged in.