Ok, so you've probably figured if you've read some of my recent twitters that I've become ever so slightly addicted to openstreetmap mapping.
I'm so involved I co-organised a mini mapping meet in Farnham (Surrey, UK) recently and dramatically improved the coverage and detail there; I took one of the most involved sections around the town.
It appeals to the slight OCD tendency many geeks have and helping out with OSM really feels like I'm contributing to a useful resource that many other people will use, so there's a big feelgood factor; plus it can be quite social and gets you exploring areas you wouldn't normally go to otherwise, even finding paths or features nearby that you wouldn't have discovered without the OSM motivation.
At the moment, the data isn't complete enough for the UK to be consistently usable for routing, but Central London is generally mapped better than google-maps with many details that you don't get with other maps (eg. amenities like recycling, telephone boxes and postboxes). The OSM Foundation reckon they'll have all the UK roads mapped by the end of 2009. Most of the work is being done by the few committed mappers with lots of time on their hands, but in the 'long tail' methodology, many many thousands of people have updated the places and names near them that they know through local knowledge. Maybe only one or two updates each, but its the sum total of those updates that makes the real difference.