http://www.mapstraction.com/
Mapstraction is a library that provides a common API for various javascript mapping APIs to enable switching from one to another as smoothly as possible.
http://www.openstreetmap.org/
OpenStreetMap is a free editable map of the whole world. It's a wiki-like map which users can edit. You can upload GPS traces of areas and add street names, etc. Eat your heart out Google Maps.
