OpenStreetMap 

OpenStreetMap, despite having “map” in its name is more than just a map, and certainly more than just a street map. It’s a collaboratively authored and edited geodatabase. You can enter anything that is objectively verifiable “on the ground” into it, and conversely you can find almost everything in it, though without any claim of accuracy, completeness or up-to-date-ness. (Though if you notice anything to be inaccurate, incomplete or out-of-date, you’re free to improve it.) The data for the complete “planet” can be freely downloaded and analyzed, or used to build your own maps, navigation, or what-have-you. There’s a whole ecosystem of third-party services, tools and derived works (such as maps) that has developed around that.

The data model is unlike the others commonly found in the GIS world, which makes analysis of OpenStreetMap data with general-purpose GIS tools cumbersome at times.

Disclosure of conflicts of interest:

I am:

• a user and contributor of OpenStreetMap
• a member of the OpenStreetMap Foundation (OSMF)
• a board member of the Swiss OpenStreetMap Association (SOSM), the Swiss local chapter of the OSMF

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near as complete as Google’s. Also, because OSM relies on free satellite data, a lot of the aerial imagery is very outdated.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.