When I was evaluating engines for Hype Pro's [1] physics system, I compared based on file size, commit/project activity, how easy it would be to modify and integrate, and the license (for obvious reasons). I looked at Matter.js, Verlet, PhysicsJS, p2.js, JPE, Newton, and box2d-js. While many have their respective strengths, overall Matter.js stood a notch above the rest. The code has been easy to work with and Liam has also been responsive in fixing bugs or accepting patches. It is truly a gem of a project.

[1] http://tumult.com/hype-pro/