SkyCharts v1.4.4 and greater is using RouteMe to display the maps.
Backend tools on linux use GDAL to create the maps.
Some icons are from Wikimedia Commons, like the P51 airplane silhouette, and states.

Other icons are from and

Satellite Tiles Courtesy of MapQuest 
Portions Courtesy NASA/JPL-Caltech and U.S. Depart. of Agriculture, Farm Service Agency.

Active Checklist uses a really nice 3D model of a PA18
rendered in 2D:

ONC charts from the University of Texas:
“Used by permission of the University of Texas Libraries, The University of Texas at Austin.”

Map tiles by Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under CC BY SA.

New continents from Wikimedia commons:
LocationAfrica” by see above – map adapted from PDF world map at CIA World Fact Book. Licensed under Public Domain via Wikimedia Commons.

SkyCharts XC and LightBox Extra for iOS/Android/FireOS is built using:

Big thanks to BinaryCart for their demo of the tools.

SkyCharts XC uses Leaflet for the maps.