The Google Maps API is listed as the most popular API in the ProgrammableWeb API directory. Because of its ease of use, plus the infinite possibilities for hacking together mashups, has made it the most popular API by far, way beyond the Twitter API and Youtube API.
In 2012 Google Maps is still a ubiquitous presence when it comes to mashing up data from APIs, but a new mapping player has stepped up demonstrating what the future of mapping, mashing up data from APIs, and city exploration can look like.
UpNext Maps has launched two new interactive mapping applications, UpNext HD Maps and UpNext 3D Cities, which allow iPad and Android tablet users explore discover cities around them in a new way. UpNext Maps has pulled rich places, venue, restaurant menus, reviews and transit information from APIs like CityGrid (disclosure: I contract for CityGrid) and Foursquare APIs, and presented in a new way that makes exploring cities fast, smooth--with rich visual detail and personalized.
There are Android and iOS versions of UpNext Maps, demonstrating a multi-platform way of mashing up rich content from multiple APIs and taking it beyond just mapping, and delivering a 2D and 3D experience that transforms the way we explore the world around us.