Nobody's mentioning old commercial games that were later released by their publishers as open source. One example is the classic combat flight sim "Rowan's Battle of Britain", initially released in the year 2000. You can find it packaged to run under linux/wine, along with other free games, here:

https://github.com/sim-museum/esports-for-engineers

the source code is here: https://github.com/gondur/BOB_Src