The Chaotic Workshop Modkit
The Chaotic Workshop Mod kit provides an SDK to code and create your own Items and Levels for the use in Chaotic Workshop. The game uses the Farseer Physics Library to create a physics sandbox.
Mods in Action
Get Chaotic Workshop
You'll need a copy of Chaotic Workshop to use the SDK. You can download it from Steam below:
You can read the Wiki here to get started. As well with this sample, the monogame framework and a multitude of examples for Farseer online such as it's own test bed, it's incredibly easy to get started today creating your own custom physics sandbox today!