I have used Elance.com and Freelancer.com. Elance would be better than Freelancer, in terms of high paying projects.
Rentacoder ( now vworker ) is not at all good... they are heavily biased towards buyers and also dont understand how software are being made these days.. they have many shortcomings and above all their interface is a thousand years old ... working with a client on rentacoder is like a master slave relationship where the coder / freelancer have to work like a slave.. Elance is one of the best and they are constantly evolving and improving...