Actor Allusion: In an early cutscene that finds the Boss watching the news shortly after escaping from prison, pay close attention to the news ticker on the bottom of the screen. It's mentioned that Benjamin King wrote an autobiography that would be made into a movie, where he'd be played by Michael Clarke Duncan. Michael Clarke Duncan is King's voice actor.
In one of the Heli Assault missions where Shaundi makes drug deals, she'll swing round a coffee shop. "I ain't doing a damn thing until I've had my Boston Cream."
Completing a mission line earns you a number of gang-related rewards. For the Brotherhood, that includes Maero's outfit, the description of which claims "I guess today really was a good day to die." Michael Dorn is Maero's VA.
The second game has a fair amount of references to anime and manga, particularly as far as the Ronin are concerned. Three of the bikes in the game are named Kenshin, Tetsuo, and Kaneda; the latter two are references to AKIRA, the former, possibly Rurouni Kenshin. Shogo Akuji and his father Kazuo are named after Shogo Kawada and Kazuo Kiriyama from Battle Royale. Finally, both Shogo and Kazuo are voiced by well-known anime VAs: Yuri Lowenthal and Steve Blum, respectively.