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

Sbox 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, Sbox sounds like a robust tool – it's got features and systems for visual scripting, open world terrain, and creating shaders.