The Frieza Force's Arm Cannons (Dragon Ball Z) amplify the user's ki and fire it at a level suiting their battle power, the power of the beam being based on the power level of an opponent measured by the scouter that the cannon is hooked up to.
In his battle against Sora, Lexaeus (Kingdom Hearts series) was able to increase his power when his health dwindled, adding to his impressive strength and allowing him to overpower Sora.
After powering up, Devil Shredder (Mutant Turtles: Superman Legend) becomes powerful enough to deflect a tornado with his bare hand.
Yellow (Pokémon Adventures) can synchronize spirits with her Pokémon, boosting their power greatly.
Robocop (Robocop) adjusting the power levels of his tactical ordinance grenades.