My small startup, Tasty Labs, has recently launched human.io. Human.io provides a simple way to allow a publisher to turn a passive audience into a mobile army of participants. This allows publishers to easily create missions and activities to get people involved more directly than just reading stuff on a screen. If Twitter is HTML, then Human.io is CGI.
Human.io lends itself to small, simple tasks: Vote on an item, take a picture of a storefront, etc. It allows you to script with humans as easily as you would script with software. It also offers easy access to the sensors on the phone: GPS, camera, and so on.
The architecture borrows from Twilio: The developer sends UI widgets to the client, receives HTTP callbacks when their users perform actions, and then responds with more UI widgets. We can also use a long poll so you can easily run apps from behind a firewall, on your laptop, or otherwise without opening a port on the firewall.
We also built Photo Scavenger Hunt on the platform as a fun little demo. I'm going to give away an ELPH camera to whoever's winning by tomorrow.
Update: Tasty Labs was acquired by Walmart Labs in May, 2013.