Google’s mapping services are excellent, extending the basic map concept with aerial views, street view and even the ability to create your own maps. However, the Open Source project, OpenStreetMap goes a step beyond that by being created through user submissions.
Initially that meant it was very sparse but the data has built up over the last few years so that it matches or exceeds the detail on other maps in some areas. Heavily populated areas often have minor footpaths and features missed by something like Google Maps (and other services); even ‘middle-of-nowhere’ locations can have surprising levels of detail if a map geek with suitable tools has happened to be passing by.
You can also edit the map, allowing other users to benefit. You can create public map views in Google but here you get to work directly on the map itself. This is kept under careful control (eg. no anonymous edits and an interface that seems designed to put off anyone seeking a bit of fun by rerouting roads!) but works well when you get used to it. I was able to add a node for the reception desk where I work and then send details to a couple of people I will be meeting with soon. That is likely to be a useful landmark for other people, too, so doing a little bit of work to meet my need will eventually also serve others.