I happened to work on some of the decentralised computing and networking projects in Barcelona. Guifi.net is the largest community network at the moment that among others lets users share and publish their content including internet access [1]. Last year I worked in Clommunity [2], an effort to build the first large scale community cloud. Guifi.net users can just install the Clommunity distro [3] on top of and use its service discovery and decentralised cloud management tools to contribute and benefit from marginal resources. I think it's a cool idea that we will keep revisiting in the long future...

[1] http://guifi.net/ [2] http://clommunity-project.eu/ [3] http://cloudy.community/