The Wild Bunch (1969): Sam Peckinpah's dark, bloody western is bookended by two of the most spectacular gun battles ever filmed, bringing home the costs of violence with their realistic depictions of collateral damage.
Robocop (1987): Generally considered one of the most violent films ever made, Paul Verhoeven's futuristic satire uses its high body count and gruesome methods of death to comment on the increasingly violent nature of American society.
The Naked Gun (1988): The spoof that launched a thousand imitators parodies hard-boiled police thrillers and includes a scene in which two characters engage in a shoot-out from behind trash cans that are only a few feet apart, and when they run out of bullets, throw their guns at each other.
Face/Off (1997): John Woo turns two-fisted shootouts and Mexican stand-offs into an art form, and manages to make one of the most preposterous action movie plots ever into something exciting and sometimes even touching. Plus, the guys switch faces!
The Matrix (1999): The term "bullet time" originated with the Wachowski brothers' balletic sci-fi/martial arts hybrid. Dig the scene where hundreds of bullets literally rain down in artful slo-mo.