I currently use peopleperhour and upwork (previously odesk).
I use them for the following
Upwork for cheaper, lower skilled jobs - had mixed experiances but found some good regular talent.
People per hour - for more specific, higher skilled tasks, however the quality of submission has been going downhill of late, they have also raised their rates and put barriers in place for people to easily sign up, so I would imagine freelancers are also looking elsewhere now. However, saying that, I have still found some excellent people, it just takes longer, more filtering and there is less of them. I would also avoid using their service where you just post the job and they choose a freelancer, I can't remember the name of it off the top of my head. The people they sent me had no idea what they were doing.
I use to use rentacoder for php at one point but the skills pool and traction had gone the last couple of times I tried, they may be better now. Haven't tried them for over 3 years now.