Fully open source and open content is a pretty strong constraint. Nethack and Angband are probably the ones I've spent most time with. I also had a phase where I spent time with springrts and the games built on top of it.
A more recent one is Remnants of the Precursors, a rather cool remake of Master of Orion 1. Haven't played it as much, though.
If we limit the restrictions to just open source, maybe Quake? And Civilization IV is not fully open source, but all the game code is.
A more recent one is Remnants of the Precursors, a rather cool remake of Master of Orion 1. Haven't played it as much, though.
If we limit the restrictions to just open source, maybe Quake? And Civilization IV is not fully open source, but all the game code is.