X3G
From Just Solve the File Format Problem
X3G is a file format used by MakerBot, with firmware 7.0 and up. It replaced S3G, used in earlier firmware versions. It contains instructions in GCode telling the 3D printer what to do.
Other formats used by MakerBot hardware and software: MakerBot instruction file, STL, S3G, Thing file, and Wavefront OBJ.