Wouldn't you like to know if she answers on whether she would consider an open relationship or if she enjoys giving oral? It's very popular but the quality isn't great, so you have to sort through many profiles.
Craig's List Personals This used to be the the ultimate personals site, but now it's loaded with so much spam and fake ads that it's hardly worth it any more unless you're very patient.
Categories include romance, casual encounters and more.
Ok Cupid This free dating site has exploded onto the scene and it offers a cool option.
They also have a blog about ways to use the dating site, and a recent post on what types of profile pictures work best is a great example of how they offer real advice based on actual usage by real people.
Finally, if you're a little older and like looking for younger woman, keep in mind they won't let you see any profiles of a woman that's more than 14 years younger than you. Adult Friendfinder This is another huge adult personals site.