You make money per lead/action/affiliate sign-up, so why would you want the signups to be free. Or do you mean that you want yourself to signup for free? In that case Friend Finder would be OK - a bit saturated, but still possible. That is, unless you're trying one of those craigslist, auto responder ads thing. You can also look up for other affiliate programs on azoogle. Hope this helps
Let me be clear. Which dating site pays the highest for a lead. Example: Somebody goes to the site and signs up for free and I get paid for that lead.