Sketch
From Just Solve the File Format Problem
Sketch (sometimes called Sketch App) is a proprietary vector graphics editor for Mac OS X, developed by a company named Bohemian. It has a native .sketch file format.
Disambiguation
There are a lot of other things named Sketch, such as Skencil SK and EPOC Sketch.
Format details
Apparently, Sketch formats through v42 are based on SQLite. Later versions are based on JSON.
