Microsoft announced today that app developers can now download the Bing Map App SDK and start building applications to submit to the Bing Map App Gallery.
Developers can download the maps SDK at connect.microsoft.com/bingmapapps. Then once you start developing your apps, you can test the app in the new maps developer sandbox.
Make sure to check out the Bing Maps Gallery to get new ideas and start building your own mashups.
Of course, Bing is not the only one with map APIs. Google has a Google Maps API, Yahoo has a Yahoo Maps API and so does MapQuest.