This app allows you to navigate freely in an empty 3D space and use your mouse and keyboard to build cube-based 3D shapes. As you're adding those elements to the scene, the program will quickly optimise the new geometries by reducing the number of vertices needed to render them and merging them with other previsouly added elements.
The master branch plays with random terrain generation, by using fractal noise made from multiple perlin noise layers composed together for a more "natural" result.