OkCupid! is great, IMO. You it's built around tests and questions which profile your personality, and allow you to specify how important you consider certain opinions in a mate. So it goes even further than simply getting you to specify a group of personality traits, it determines them from what you tell it.
I find the questions to be a little too ambiguous in most cases, or too simplistic, but that's the problem with 4-option multiple choice. And you can easily blog about any question if you want to express an opinion the available answers don't allow you to.
The site is great for finding different kinds of relationships too. I actually haven't found a partner yet (not so many people in Australia on the site), but I've made a few very good friends.