We're looking at different freelance marketplaces where startup founders turn to in order to hire the resources they need for very early stage dev work, marketing, and other tasks. Specifically stuff that can be done cheaply.
The best ones that I have used are as follows:
O-Desk: https://www.odesk.com/. This is good for more complex tasks that are still relatively cheap to implement.
Fiverr: https://www.fiverr.com/. These are small jobs that cost $5 bucks per gig. Good to get small stuff done fast.
I hate to say this, but this is how I found a great outsourcing partner in India. I spoke to my friend who recently moved to the U.S. from India and ask them if they wanted to be my "interpreter" to find a good outsourcing partner for additional tasks. They actually knew people doing exactly what I wanted to get done and they were able to connect me to a great small firm in India that has great and ambitious contractors for hire for only $6 per hour.
Sometimes the best way to do it is ask an Indian or Chinese friend :).
This doesn't specifically answer your question, but a lot of people steer clear of "the big sites" like eLance or oDesk because they're worried about getting low-quality code back or having other issues with the deliverables.
If the kind of work that you need done is fairly vanilla web development (ie not scientific computing, super high performance stuff, specific algorithms, etc) I've found that you can actually get high-quality code back from the freelancers on those sites for very cheap.
The important thing is to be very specific with them so there's as little ambiguity as possible. You'll need to write detailed product and technical specs yourself, as well as create mockups of what the pages should look like. These are the components that require the most design and careful thinking.
Once you've fleshed all of that out, it's easy to hand off to someone cheap and and have them crank through it quickly. They'll usually appreciate that their project is so well defined.
So while there are definitely some horror stories that come from the "big" freelancing sites, they can definitely be a great way to hire freelance resources, if you know what you need to have built very specifically.
I can only talk about Odesk which was good, Mokriya I heard fantastic things and a friend used augmenteddevelopment.com and have good references.
I also know some freelances that are really good (but DM me)
I being an app developer get hired more via my own site (www.agicent.com) than upwork (I've to bid a lot to get small of work on upwork). Startups find us via social forums like this, or via google search or meetups and then discuss their App ideas and then we offer our best app development services and if all goes fine then its development.