The sequel to Garry's Mod releases this month, and Valve will let you publish your creations as standalone Steam games, like dafty Finn sim My Summer Cottage

S&box is an upcoming open source game development platform, built using a modified Source 2 engine. Out on 28th April, it's the work of the very same Garry Newman behind the legendary Garry's Mod, who's been working on a spiritual successor at his company Facepunch Studios (also the developers of Rust, and originally known as Team Garry) since 2015.

Inasmuch as I am qualified to judge, S&box sounds like a robust tool – it's got features and systems for visual scripting, open world terrain, and creating shaders.