I hope it has not been done before.
I had this thread Idea after reading this thread.
Let's do a list, then!
- In Lemmings, some levels where using music and artwork from some of their other games.
- In Duke nukem 3D Atomic (pc / mac) there is too many references to list them all, but here are a few exemples: a hidden chappel where you can kill the Doom character, a police station with Harry Calahan's desk, a hanged Indiana Jones, "Wanted" posters with Unabomber's face and also Unabomber's flat, 2001's Monolith, etc etc...
That's all I can think of. Add the stuff you know!