Intro from http://wiki.openstreetmap.org/:
OpenStreetMap is a project aimed squarely at creating and providing free geographic data such as street maps to anyone who wants them. The project was started because most maps you think of as free actually have legal or technical restrictions on their use, holding back people from using them in creative, productive or unexpected ways.
OSM is a highly regionalized effort and information is available in English, Czech, Deutsch, Italiano, Español, Nederlands, Slovak, Slovenščina and Svenska (check the Wiki for current status). OSM bridges the gap between software and geodata that is inherent to most other implementations. This is of great interest to OSGeo's mission of which Geodata is an important part. Bridging the gap has to be paid for with a few caveats and OSGeo has a potential and the means to help resolve some of these. The more obvious issues are interoperability with standardized software, scalability issues and inconsistent semantics.
OSM will also be at the LinuxTag 2007 a good time to discus, meet us there.