need rating system that allows anonymous and tracks by cookie, not IP

I was asked to quickly set-up a rating system at work so I used fivestar, but the problem is all the people doing the rating are in-house and share the same IP address. Fivestar just sees each new rating as an anonymous rating from so-and-so IP and overwrites the last rating for that piece of content. (ie. John's rating overwrites Jane's rating which overwrote Tom's rating) The end result is, people are rating the essays but only the last rating is actually recorded in the DB. (that means, each piece of content only ever has one rating)

So, is there a rating alternative to Fivestar that recognises anonymous users by setting a cookie rather than looking for a unique IP? It needs to be for just anonymous users and allow them to vote just once. (unless they're sly enough to clear their cookies and then I can't really do anything)

Any ideas would be appreciated before I start rolling my own module.

Thanks,
Märt

UPDATE - I dug into the guts of the votingapi.module and that was where anonymous users were being recognised by IP. ['vote_source' => ip_address()] I did a quick hack and changed the vote_source field to save their session_id instead of their IP address ['vote_source' => session_id()]. The end result is I'm still using Fivestar, but anonymous users (all with the same IP) are recognised and have their ratings tallied as different users. It's not fool-proof, but it works for my purposes. Now I need to see if this is something the votingapi module maintainer would consider a useful option to have. (ie. differentiate anonymous users by session id instead of IP)