As has been mentioned elsewhere on the boards, okcupid is at least accepting of polyamory. You can list your status as available, single or seeing someone (there might be some other options, as well). I've stuck to just messaging people that were available, meaning they have some sort of relationship, but would be open to another, or those that actually mention poly in their profiles. I live in a major metropolitan area, but I've had no shortage of choices.
Also, the user interface is good and its matching engine works really well, as far as I can tell. Being free is always a plus, as well.