Are you looking to just buy the NAS itself and put your drives in, or buy the NAS/external enclosure?
I have a Dlink DNS-323 and have been happy with it. I know they make a slightly cheaper model too.
I think there are a number of them available that are just 1 drive enclosures as well if that suits your needs - I think Buffalo, Thecus, and Synology make some nice ones.
Also, I think the Western Digital WorldBook is an external drive/NAS.