More then 20000 users have contributed to the growth of the project OpenStreetMaps. Using wiki principles the project collects information about streets, buildings, rivers, landscapes and everything that is usually displayed on maps. The agency Itoworld has created a video that animates the coverage of the gathered data. Really beautiful:

So you say: What do I need another free map service for? Is Google Maps not enough?.
Check out this comparison by Bodenseepeter [in German, illustrated]. He analyzed the extend of the two map services usinng ten examples worldwide. For example Cape Town:

Left in Google Maps

Wikipedia:

OpenStreetMap (OSM) is a collaborative project to create a free editable map of the world. The maps are created using data from portable GPS devices, aerial photography and other free sources. Both rendered images and the vector dataset are available for download under a Creative Commons Attribution-ShareAlike 2.0 licence.

via T3N magazine Germany