Appleseed is the second renderer which will be implemented and supported here. The developer François Beaune created a really cool renderengine. Have a look at AppleseedHQ website for more infos.
One of the very best parts of Appleseed is that it comes with a complete API. This encouraged me to try to implement my very first complete renderer API implementation with renderview feedback and other cool things. And it seems to work. I'll post the news as soon as I have a complete working example.
The sources of the plugin are available on github.
For any questions/discusstions, I opened a group on google groups: mayaToAppleseed.