I don't like craigslist. If you're in college facebook is probably your best bet. You can easily meet tons of people through that. Just don't be obvious that you are looking for hook-ups. You'll come across as creepy or desperate. This is a mistake that MANY guys make. Ask for (or obviously go after) sex directly and you're not going to get it. Pretend like it's not something you're interested in, and the sex will follow. Stupid, but true.
Hot or Not used to be really good but now it's no longer free. The free part of AFF (the chat rooms) is an okay place to find hookups but it's mostly old ugly swingers in there. Not many people college boys would be interested in. The pay side of AFF is completely worthless.
I've met several girls at gothic personals.com It's a free site. Good if you like goth girls.
Other than that, not very many free sites out there that are worth much. Plenty of dating sites that do work if you're not so cheap you won't drop 20-30$ on a membership, though. Most of the free ones cater to young teens.