The following is a list of all the NES games I've finished, in alphabetical order. The count currently stands at 56 games:

A Boy and His Blob
Alfred Chicken
Amagon
Astyanax
Bad Dudes
Baseball Stars
Bionic Commando
Blaster Master
Castlevania
Castlevania 3: Dracula's Curse
Commando
Contra
Cyber Stadium Series Base Wars
Donkey Kong
Donkey Kong Jr.
Double Dragon
Double Dragon 2
Double Dragon 3
Dr. Chaos
Dragon Warrior
Godzilla
Karateka
Karnov
King's Quest 5
Kirby's Adventure
Kung Fu
Lemmings
Life Force
Little Nemo the Dream Master
Low G Man
Marble Madness
Megaman
Megaman 2
Megaman 3
Megaman 4
Megaman 5
Megaman 6
Metroid
Ninja Gaiden
Predator
River City Ransom
Rush 'n Attack
Super Dodge Ball
Shadowgate
Super Mario Bros
Super Mario Bros 2
Super Mario Bros 3
Teenage Mutant Ninja Turtles
Teenage Mutant Ninja Turtles 2
Teenage Mutant Ninja Turtles 3
Ultima Exodus
Wizards & Warriors
Wizards & Warriors 2
Wrate of the Black Manta
Wrecking Crew
Zelda II: The Adventure of Link