The OpenStreetMap (OSM) project is well on its way to producing a free, user-editable street-level vector map of the world. OpenAerialMap (OAM) is similar in scope, committed to building a free, bird's-eye photographic map of the world. But it faces a unique set of challenges.
For covered areas, the experience is similar to that of proprietary tiled geodata services like Google Maps, MapQuest, and Yahoo! Maps. As you zoom in, the server replaces tiles with higher-resolution shots when available, and falls back to enlarging smaller tile images when necessary. The chief difference is that -- as a free service -- the project cannot afford to purchase aerial photos from the commercial mapping services.