If you've used http://edinburgh.cyclestreets.net or http://glasgow.cyclestreets.net (and plenty of other places) you'll have used OpenStreetMap - the digital map put together by anyone/everyone (maybe you).
A lot of the work is done by people working alone, but occasionally there are parties. There's one on Saturday May 22nd, 10:30am - 4pm for the Queen's Park area of Glasgow.