I have been adding data to the Open Street Map project since last Autumn. It is interesting and satisfying but I haven't kept any blog about it, so here is that blog ...
Open Street Map (OSM) is a Wiki project to create a free-to-use map of the world. People contribute a bit or a lot by any means they can, so long as they data has no copyright strings attached. You can find OSM at http://www.openstreetmap.org My user name is chillly (three l s).
You can trace over aerial photos using a tool provided called Potlatch. These are Yahoo photos that are of varying level of detail depending where you looking at. It's useful, can be accurate (partly depends on the Yahoo detail) and needs nothing but a browser.
You can also gather data with a GPS device, upload it and then use the tracks to draw the detail to add to the database. I find this much more interesting since you actually have to visit the places you map. We (Jean and I) visit a place with our GPS and camera either on foot or by car and cover the roads, footpaths, byways, car parks or whatever, letting the GPS record our route and recording place names, street names and other notable items such as pubs, churches, schools, shops, etc with the camera. This lets us add much more detail with Potlatch, but only for places we can visit.
Other people have other means of adding data, such as loading publicly available data, some of which seems very good - some not so good. Some work on their own area, others enjoy adding distant parts of the world that no one else is working on yet.
It is surprising how things turn up that you didn't know were there and how much you can deduce from looking at something from afar.
We are working on the East Riding of Yorkshire at present and Kingston upon Hull, though the odd foray elsewhere is always possible. I intend to summarise the work we have done so far and then keep a track of progress. We have a Wiki page about East Yorkshire to keep track of things, but it is not descriptive.
Another important part of the process is rendering which turns the raw data into a map which is useful, but much more of this another time.