There has been a flurry of new mapping-related APIs added to the API database here. These include MetaCarta, Mapstraction and Map24. This brings the total number of APIs with some mapping-related characteristic to 23. You can see them all here.
This includes well known APIs like Google, Yahoo! and Microsoft Live maps; geocoders like geocoder.ca and Ontok, layered services like Wayfaring and Feedmap for blogs; and a couple of farther out ones like NASA satellite imaging.