We cycled all around Costa Rica, and we wanted to visit communities and projects, but we found that many charge amazingly high amounts of money to stay and/or volunteer with them, and as we did not want to support this practice, we did not visit many.

We did spend a week in the mountains near Orosi, a town close to Carthago and San Jose, where Frank (USA) owns Hotel Reventazon and a large piece of land on top of a mountain with a beautiful waterfall and amazing vistas.
When we stayed there we a few half finished earthship structures, an open air kitchen, shower, greenhouse, some tipis and plenty of garden.
And we stayed for free ; )

More information via http://eco-sanctuary.blogspot.com