DIY City  http://diycity.org/ 

If you are interested in social media quite widely defined, public space, local governance, information feeds, mashups type of stuff for the public good.

The London group only had 3 members last time I looked.

The Britglyph project is a collaborative public art project across all of Great Britain. It relies on members of the public to participate in the creation of the worlds largest artwork. Log onto http://britglyph.com and identify a location near you. You then need to go to there, take a photograph and then upload the image to Britglyph. If you are interested in taking part click on the link above. @julianlstar