Marathon Aleph One
Way back when, Marathon was a great game for the Macintosh. Released at about the same time as Doom, it featured an intriguing story as well as exceptional gameplay. Future versions upped the ante in the story-telling realm considerably; some still count Marathon among the greatest of games purely for its story. I wasted a considerable number of hours playing the trilogy in college.
Before its maker — Bungie, of lamented memory — was bought by Microsoft, they freed the source to Marathon, and after Marathon, Marathon 2 and Marathon ∞, we now have Marathon Aleph One (math geeks wil recognise that Aleph One is the power of the continuum, which is larger than simple infinity …). Very, very cool.
04 February 2018: updated URL