The Community Mapping Network (CMN)provides an online mapping application that allows folks in British Columbia, Canada, to create and edit information about environmental resources in their areas.
The application is used for sustainable city planning efforts and other types of environmentally-sensitive decision making.
Perhaps more interestingly, the CMN also hosts a directory of international projects, which you can contact and volunteer with if you're so inclined. These are mostly intended to "collect natural resource information about fish and wildlife species and sensitive riparian, fresh water and marine habitats."
It's a Canadian grassroots effort, and it's a wonderful thing to see. I especially like their broader mission to empower communities environmentally:
Mapping typically takes place in rural and urban areas to assist planning sustainable communities and should also be used as a vehicle for empowering community conservation and stewardship of natural resources."
I love this site and their model (although some of their Macromedia ColdFusion mapping applications didn't work on my computer.)
I believe that this type of project might be feasible for other types of community projects or collaboratives. Perhaps GPS cellphones and some server-side GIS could make a popular urban system investigating, perhaps, an environmental justice problem.