I've been waiting for
Crabgrass
for years. I want to set up tasks and keep track of what my mates are
doing without struggling with Gantt charts and all those tools for
MBAs; looking at the grid of lines on a spreadsheet makes me think of
jail. Crabgrass offers all the elements of social networking that are
actually useful.
A friend who has been trying it out, Gabriella Coleman, says it's already stable and being used by non-profits and activist collectives around the world. It has recently received an infusion of funding to polish usability. And it's under the Affero GPL!

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Now you have also WiserEarth.org; which is also open, free andvery powerful !