Open Street Map – if you’re going to edit it, you should look at it first!
Much of the power in it is in the layers you can add over the top, visualising various sets of information. You can also add background layers – like arial imagery from Bing.
To edit it, you need to create an account. Then? You needy to both do the surveying and add it into the system. Some people take notes and sketches, some people use apps.