Impact provides 2 types of [collision detection](http://impactjs.com/documentation/collision); static and dynamic collisions. Both of which are easily integrated into game development.
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Starling is just 12k lines of code and doesn't try to do everything - but what it does, as efficient as possible.
Pro Free and open source
Starling is available for free with code available on GitHub.
Pro Strong community support
Forum is always active with knowledgeable developers and with lot of inside info, and post mortems.
Pro Excellent collision detection system
Impact provides 2 types of collision detection; static and dynamic collisions. Both of which are easily integrated into game development.
Many plugins are available, including one called Impact++ which adds features like pathfinding and dynamic lighting.
Pro Level editor
Built-in map editor with support for tiling, collision layers, and actors.
Pro Extensive documentation available to support development
ImpactJS has an active user community that busily produces tremendous resources including videos, books, tutorials, walkthroughs and more. There are also comprehensive class documentation available.
Pro Truly cross-platform allowing developers to build for anything
Extending the reach of a game developed with ImpactJS is easy due to the cross-platform nature the framework. There are Considerations around performance, resolution and audio, however all can be appropriately addressed when in development. Works in the browser via Canvas, even on mobile. Easily translatable into a packaged app.
Con Early in development
Con Not being maintained
Impact is not being maintained anymore. Last release is from 2014.