The only other that I'm aware of at the moment is Tweaky.com.
Tweaky appears to focus on front-end development while I see a back-end task or two at Bountify.co.
Another difference is the minimum price. Tweaky is at $39 and Bountify is at $1.
Are there other big differences between the two?
I would hesitate to jump into a marketplace with low minimum prices because I have been groomed to 'get what I pay for' with development. It's hard to imagine that $39 is sufficient for a lot of tasks let alone $1.
As a person currently in the market for developers:
Finding a front-end rock star went well although it was slow going. We filled that position within a couple months and have asked that developer to source to Tweaky (and to check out Bountify) for spillover until/if/when we need to hire another person in-house. Our in-house developer would audit/implement changes coming from Tweaky/Bountify.
Finding a back-end/systems engineer rock star is turning out to be a bigger challenge for us and at this moment I would find a lot of value in a verified systems engineering marketplace to source work to. I find Bountify appealing in that sense although I would wait for a high quality validation procedure before sourcing systems work to a 3rd party. Even then, I would only allow access to a test environment with a requirement for instructions on rolling the changes live so I could bring in somebody who has worked closely with me to audit and implement that solution.
The bottom line for me: I am very much in favor of this type of specialized marketplace and am glad to see Bountify.co enter the arena.