Upwork : I use them, but hate
them. They are a classic study in the destructive effects when two major competitors merge and end up 'owning the market'.
When eLance and oDesk were separate, I preferentially used eLance and was very happy with the service, but the new Upwork service is dreadful, and their website beset with bugs and problems that their own customer support people don't understand and can't resolve - most recently, I was unable to end contracts with people who were working for me!
Plus I got a note yesterday that they are totally changing their fee structure (and, here's a hint, the 'change' is not an improvement for employers).
But who to use instead? There's a great gap now, and no-one clearly available to fill it, particularly for developer type positions.
Fiverr - have used them from time to time, and they provide clear proof that 'you get what you pay for'. Template designs rather than custom designs, and the design I bought (costing considerably more than the $5.50 entry price as many things invariably do) and which I was promised was all custom artwork ended up being found to be clipart and in use dozens of other places!
Use them with caution, but for fast and nasty proofs of concept sort of things, and idea generators, maybe a good place to start.
DesignCrowd - I quite like this service and have used it a couple of times; indeed, the biggest 'problem' with them is getting too many designs to choose between! That is probably a good type of problem to experience.
I've dabbled with others from time to time in a never-ending quest to find people who are simultaneously the best and the best value, but every time come back to the appreciation that, sort of similar to dating sites, there's only so much that these sites can do, and beyond that, it is up to us to evaluate, choose, and then control and manage the people we hire.