Over the last day or so, I've been working on putting together a new event in London, Operation Cowboy. The plan is for it to be the weekend after next, which is very soon, I know.
http://lanyrd.com/2012/cowboy-london/ https://wiki.openstreetmap.org/wiki/London_OPC2012
Operation Cowboy is a plan for an all-night OpenStreetMap editathon (mapathon!) focussed around improving OpenStreetMap for the United States, an area OpenStreetMap is known to be not quite so good (compare San Francisco with London: on OpenStreetMap, damn near every pub, bar and shop in central London is on the map, in San Francisco, it's not nearly as good).
Though the best mapping we can do generally involves getting a GPS out and walking or cycling the streets yourself, there's plenty of work that can be done to improve OSM from your armchair.
You can read more about Operation Cowboy at: https://wiki.openstreetmap.org/wiki/Operation_Cowboy
Part of the point of the Operation Cowboy event is to be a place where Wikimedians and others who haven't played around with OpenStreetMap and want to learn can come along, learn how to set up an account and start editing. Though we'll use the US as the focus of the event, the skills people learn improving the US map will be applicable to improving the map for their local area in the UK.