ggmap allows to visualize spatial data by combining data from Google Maps, OpenStreetMap, Stamen Maps and/or CloudMade Maps. It adds functions to access Google's geocoding, distance matrix and directions APIs.
CloudMade Maps are derived from Open Street Maps, but allow a user to define his own colors and lines etc for roads, waterways, landmarks etc. However, in order to use these, an API key ust be obtained. (It seems that as of May 2014, this is not an option anymore, anyway).
Stamen maps are also built using Open Street Map data.
ggmap uses the layered grammar of of graphic implementations of ggplot2.