OKCupid.com is the best I've seen. And it's free too (completely). They have a cool matching system that asks you questions. Based on your response of what your answer is, how you want the other person to answer, and how important their answer is determines the relationship %, friendship %, and enemy %. Once you answer I think 500 questions, you can start submitting your own to be added to the list. When I joined a year ago there were only like 1800 questions, now there is over 4000. Also, the more questions you answer, the more different personality traits will show in your profile (like you could be less complassionate, and you could also be more kinky). I've answered almost all the questions, so I have a lot of personality traits that show. They also have various quizes that people and staff submit that tell you and others anything from what type of breasts you like to what Futurama character you are most like.
Adam