OSM Tasking Manager - allows a group of mappers to coordinate and divide up mapping tasks geographically.
Through this I discovered…
Humanitarian OpenStreetMap Team - OSM for a cause! You need nothing more than Potlatch to make a difference.
And through that I discovered…
Walking Papers - Offline and portable OSM mapping still kinda sucks, so Walking Papers allows printing sections of OSM on paper, then scanning your notes back in to work with them online.