Space Station 13 is something of a classic, released way back in 2003 on the BYOND service. It's being remade with Unity to have a more modern experience, as well as being cross-platform.
The team doing Unitystation just put out the first Public Alpha. Not only that, all the code is available on GitHub under an open source license. It requires Unity to build, so there's still a bit of a barrier but I still think it's great it's being made in the open.
Worth noting that since it's "Alpha", it's very much a work in progress. That said, what they're doing is awesome. They have built a launcher with a fully functional registration and login system, which gives you a server browser and a downloader for game updates to easily get into the game.
A little more about it:
Based on cult classic Space Station 13, Unitystation is a game centered around life on an outpost exploration ship in the far reaches of space.
The game is full of paranoia and lighthearted shenanigans, being a station that is as much at risk from clowns and mimes as it is from undercover operatives, eldritch cultists, changelings, demons, revolutions, and assaults from enemy corporations. Played in rounds that reset the station every time, the game features an assortment of jobs, including doctors, chemists, scientists, janitors, engineers, atmospheric technicians, security guards, bartenders and the all-important clown.
Each round’s randomly selected antagonists will try and execute plans to fight the station’s crew. Despite some unintuitive graphics and mechanics, the game can be incredibly addictive! Endless possibilities await you on the barely airtight deathtrap that is Unitystation...