Monster World 2: End of Game (all versions)
This password will work for all 3 versions of the game, but the trick below to immediately go to the final boss only works on the Game Gear versions. Once you enter the password go to the right and enter the first door. Now walk to the right and once you are in front of the well press up to enter a secret door. You are now in front of a new door. Press up to enter this door and you will be in the room with the final boss.
WE5T 0NE 0000 000 - All items, forms & equipment right before the final boss
Wonder Boy in Monster World (Master System)
Make sure you hit select to access the system menu and verify the machine is set to Master System. These passwords will only work for the Master System version of Wonder Boy in Monster World.
F72CK L6222 2A222 CK3G2 5SC2B 4222K L2K4K HDE3G 4KUU2 - 4 heart containers, first boss beat, Knight Sword
A32K2 KO2K2 2ANBZ JS4GK 7YL2Y ISANL L2KA2 P3E32 22TU2 - 8 heart containers, second boss beat, Excalibur
O72K2 MO2K4 2YNBZ JWNBX DJQ2P FSANL L2WA2 O3U32 22TU2 - 8 heart containers, third boss beat, Legend Sword
L42K2 MO2O4 2YNZZ ZYWJZ FJE3Z CSANL L2WA2 O4U32 22DA3 - End of game 8 heart containers, fourth boss beat, All Legend equipment