Airships: Conquer the Skies is a ship-building Steampunk RTS about horrible things happening to tiny simulated people. It's designed, developed and marketed by one large person and is currently in early access.
I recently implemented a combat replay system for Airships: Conquer the Skies. The motivation was to allow players to play back past fights to analyze them or simply enjoy the carnage. I also wanted to make it easier to create GIFs and gameplay videos.
The game uses lockstep multiplayer: the starting state is synced, and then only player commands are exchanged. The game state stays consistent between players because it's deterministic. This makes creating a recording system pretty straightforward: save the starting state and the commands issued, and the fight can be played back.
The cool thing about doing replays like this: because a replay is literally the same combat being played through again, the game can offer the player the option of re-inserting themselves at any time. This works by turning off the replay of stored commands, giving players control of their chosen side, and installing AIs in the opposite one. Which I think is a pretty neat and unique feature. Rewrite history! Could you have won that battle? What was it like from the other side? Would a different tactic have worked better?