Also Moonring. Not as huge as Balatro, but fairly well-known too.

Loved that I could open up balatros game files and examine the source. Pretty cool how far the author took the framework.

I had similar fun with Don't Starve when I opened up some files in the game directory and noticed they were all Lua, all using an ECS system, and easily extensible. Having never used Lua before, I made a rudimentary multiplayer server for it by opening a socket and sending game events to another instance of the game on another machine. I planned to actually start making it into a multiplayer mod but then they announced Don't Starve Together and I scrapped my implementation for obvious reasons.

LÖVR (MIT) was posted last week: https://news.ycombinator.com/item?id=41446248