Compton’s Interactive Encyclopedia brought an entire encyclopedia’s worth of information to the Sega CD (Mega CD outside of the USA) on a single disk, and included not only text articles, but also pictures, sounds, and even video, making it more like a proto-Wikipedia than just a CD-based encyclopedia.  Even though it wasn’t a game, I found it an interesting use of an early CD-based system and just how much can be stored on a single CD.  Sadly, game developers decided that having an entire CD to store data meant that they could use it for “video games” that were more video than games.  At least Compton’s wasn’t conceited enough to believe that it was in any way a game.