Openstreetmap is the open source alternative to Google Maps.
Most all of the data for the project is crowdsourced, which is both a boon and a bane. The more developed and populated an area is, the better the OSM data for it will tend to be; unfortunately, the inverse is also true.
OSM’s maps tend to be more complete and up-to-date than Google’s, but the project has no equivalent of Streetview or Google Maps’ user-submitted photos, and business information is nowhere near as complete as Google’s. Also, because OSM relies on free satellite data, a lot of the aerial imagery is very outdated.
Contributing to the project is not as easy as it could be, and there’s a rather steep learning curve even for beginners, but once one gets stuck in, it can be quite satisfying to add information–I’ve occasionally found myself filling out the map for hours at a time.