There are some open source games in the HaikuDepot, such as DOOM. There are also other open source games which might run on Haiku with a recompile or minor tweaking. There’s a recent post about the efforts of getting Postal to run on Haiku Porting Postal. So there are games in Haiku! Of course for Postal, DOOM, and possibly others you’ll need the game files, but that should be easy as the games are on Steam, and other platforms.