I bought a few things off of eBay of late, taking advantage of the 30% cashback from live.com. One of those things is Mediasonic HD9-U2LA NAS. It's a very simple NAS device that takes an IDE drive up to 1TB and has both a 100 Mbps ethernet connection and USB 2.0. Performance is not great - it looks like it sinks about 15-20 Mbps, but that's OK for my needs. Another annoyance is it uses FAT32 so file sizes are limited to 4GB. On the plus side it's fanless and quiet, relying on its aluminum casing to dissipate heat. Unlike a lot of cheap NAS cases, this one features a configurable idle period before the hard drive is powered down. For me, this is a key as the NAS is just for weekly backups run in the middle of the night. I suppose I could put it on an appliance module and power it on and off when I need it. I'll measure its power consumption later today.
Update: It uses 4 watts with the drive powered down and 11 watts with it active.