When it comes to map mashups there's no doubt that Google Maps leads the way. On ProgrammableWeb we list 1,964 Google Maps mashups, almost four times as many as any other API. This popularity makes utilities built on top of Google Maps especially interesting, such as this contextual menu control, which pops up a menu when the user right clicks.
MapQuest's recent entree into mobile developer tools is an interesting one. Choosing Android over iPhone means going toe to toe with Google, a familiar competitor to MapQuest on the mapping front. Mapping is logically MapQuest's turf. The company was helping people find directions online before Larry and Sergey were even in graduate school, let alone leaving to found Google. Of course, another way of looking at it is that Android is in line with MapQuest's latest push to open up mapping data.