Mostly I would not recommend dating sites at all. Too many professional girls use them to find customers. A walk in the park finds you real girls, and if you feel you must you can even wear your preferred clothing. That is an easy way to scare off those who don't have long term potential :-)
OKCupid.com as others have said dating sites have their pitfalls.. arguably people view them as candy shops - always looking for a new sweet taste and one doesn't hold attention for long.. but that said people do meet people, date and even marry.. The internet dating world is your oyster
Most of them. Match.com is the only one I know that forces you to pay. Plenty of Fish and OK Cupid are the big ones, but those have been mentioned. POF is interlinked with a bunch of them which all use the exact same design/ layout/ features, but cater to different groups. You can see their other sites by clicking on "Try Other Dating Sites" when you're on it. The one I used was for single parents, but there are probably a dozen or more alternatives.
Just Asking 