There's a few of them around, mostly from unknown brands. The inner workings seem to be something of a mystery. But mainly they all utilize a file server program that gets installed on your PC and they can read all the supported file types off your HD. One even has it's own HD that allows you to store files, (but it's not clear whether those files can be accessed from a PC).
The differences, if any, seem to be in the file types supported, but AVI and DIVX are typical, as are MPEG4 and audio/image files.
Some are wireless only, some CAT-5, and some offer either option. Models seem to be getting more plentiful this year.