Metroid 2: Return of Samus

[final kaoss]

60 missiles and 99 health

Enter your ship and roll left to right until both walls are hit once.

[final kaoss]

Invincibility

Begin a game and roll ten times left to right, then do a flip in the air.

[final kaoss]

Stalljump

If you do a directly vertical spinning jump and hold up, Samus stalls in midair. Perfect to use with the screw attack, since most if not all attacks will do absolutely nothing to you during it.

[final kaoss]

Different Ending

Beat the game in under 3 hours, and when Samus shows up in the ending, you'll see her in a bikini