Here's some more regional differences. Like last time, a few of these were already addressed on the wiki, but without images:
Contra and Probotector:
Title:
US: Eur:
Main Characters:
US: Eur:
Side View Infantry: Includes a running type and a shooting type.
US: Eur:
Front View Infantry: Includes a running type and a jumping type.
US: Eur:
Side View Water Commando: Hides underwater or in waterfalls.
US: Eur:
Stage 3 Boss: Color differences in the arms.
US: Eur:
Stage 4 Boss Gunners and Winged divers: The soldiers that protect the actual boss.
US:  
Eur: 
Sentry Gunner: Also worth noting, the dynamite sticks that are thrown over the background in this section share the main character's palette, so they become grey along with the main character in Probotector.
US: Eur:
Stage 5 Boss Projectiles: The mini disc things that fly out the sides have different palettes. I'm not sure why on this one.
US: Eur:
Stage 6 Boss: Made the robotic change in Probotector.
US: Eur:
Pods in Final Level of Probotector: For some reason, the power-up pods get a color change in the last level, and only the last level.
Eur: 
Ending: Helicopter in Contra, Jet in Probotector.
US: Eur:
Renegade and Nekketsu Kouha - Kunio Kun:
Title: The Japanese version's differences extend even further by having a short cutscene setting up the story for the game, where the player's brother is kidnapped.
US: JP:
   
Main Character: In the US he's just a random tough guy, in Japan he had Japanese school clothes.
US:  
JP: 
Standard Thugs: Pretty much the same kind of change as the main character.
US: JP:
First Boss: The Japanese version had another thug in a school uniform. The English release changed him to some bandanna thug.
US: 
JP: 
Motorcycle Thugs: Mainly the shirt changed, slight changes to the face.
US: JP:
Second Boss: Probably the biggest change, the US version turned him into a Mad Max-like road warrior.
US: 
JP: 
Girl Thugs: Random women replaced Japanese schoolgirls.
US: JP:
Third Boss: Big changes on the graphics of her dress, but her portrait didn't seem to change aside from the palette.
US: 
JP: 
Final Boss: His suit color was the main change. Like the boss before him, his portrait didn't change aside from the palette.
US: 
JP: 
Background Differences:
Stage 1: In the Japanese Version, it has images of a Sumo wrestler and cigarettes on the boards in the background. It is changed to a man in a hat and a picture of rootbeer for the English release. Also the locale is changed from an outdoor train station to a subway, the palettes made darker as well to reflect the change.
US: JP:
Stage 3: Japanese characters on the board become "Beer" in English.
US: JP:
Ending: The US had virtually no ending to speak of, just "The End" after the credits. The Japanese version shows the brothers being reunited after the final boss is defeated.
US:
JP:    |