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, EPOC Sketch, and AutoSketch.
Format details
Apparently, Sketch formats through v42 are based on SQLite. Later versions are based on JSON.
