No such website that I'm aware of. And I've recently searched for something similar, where people could rank images with a numeric scale (1 to 10), and then maybe sort by that scale. Looked at all the usual suspects and could not find a good one.
If I were you I'd just ship them jpeg proofs and tell them to use some viewer like FastStone. FastStone (free) has easy tagging and can display only tagged. When they are done, have them ship the tagged images back to you. Or just email a list of file names.
Make the jpegs big enough to be useful but small enough to prevent theft (or watermark).
