8. Ninja Gaiden
Cheat: Respawning 1-ups
As much as it is often an exploit, the respawning 1-up has been a practical resource for novice players for games as recent as New Super Mario Bros. U.
One of the earliest examples was in Area 5-3 of Ninja Gaiden on the NES. Go to the 3rd floor in the tower, grab the 1-up. Go back down to the 2nd floor and back up to the 3rd floor. There's another 1-up for you to grab. Repeat.