They seem to be scattered throughout the game, and most can be easily missed. What are some of your favorites?
Off the top of my head, here are some of mine.
-One of the waves in Whored Mode is called Y. Kanno. The description is "3.2.1. Let's Jam!" You control a tank in this wave. Cowboy Bebop Reference.
-In the end of Gangstas in Space, the default Male voice (Troy Baker) straight up says a line word for word from Red Faction Guerrilla's protaganist, Mason.
- The challenge for completing the Assassination missions is called "Everything is Permitted". Assassin's Creed Reference.
I know there are plenty more I am missing. I'm surprised I caught the ones I listed, to be honest.