That is a very broad question. What type of freelancer are you looking for? There are sites for graphics artists, there are sites for programmers, there are sites for writers, etc. Giving you the top 10 sites for freelance writers won't do you any good if you are looking for programmers!
A few random freelance sites that come to mind:
Jun 14th, 2013
Studypool's Notebank makes it easy to buy and sell old notes, study guides, reviews, etc.