When Power-Ups were eaten by Pac-Man, something in the level will change.
The first power-up used by Pac-Man. It turns all ghosts blue for a limited time allowing Pac-Man to eat them. These were first available on the original Pac-Man, but have been the main non power-up in most other games.
Pac & Pal
After eating a galaxian ship on Pac & Pal, Pac-Man will turn blue (rather than the ghosts) and he will momentarily be able to emit rays from his mouth which freeze ghosts.
There are also:
-Rally X Cars which allow Pac-Man to set oil smear traps.
-Trumpets which allow Pac-Man to dazzle ghosts with it's notes.
-Snowman power-ups which emit ice beams that chill ghosts solid.
-Mini Pac-Man power-ups which let Pac-Man shoot little Pac-Men at ghosts.
On the game Pac-Man Plus, there are trick pellets that instead of turning the ghosts blue, turn the maze invisible making it difficult for Pac-Man to know where he's going. These pellets are indistinguishable from the real Power Pellets.
In Pac-Man World 3, there are Ribbon Pellets. When these pellets are eaten, Pac-Man will leave a pink trail behind him, and when one end of this trail meets the other, a ring will be created that squeezes together and explodes, damaging nearby enemies. Ribbon Pellets also appear to cause everything--including Pac-Man--to move in slow motion.
In Pac-Man World 3, a Power Stomp power up is available, if this is eaten then Pac-Man will temporarily be able to do a body slam that emits vibrations affecting surrounding enemies.
Similar to Super Mario's 'Metal Mario', Chrome-Pac will allow Pac-Man to become invincible momentarily. This is available in Pac-Man World 3. A similar power-up is present in Pac-Man World 2, however, instead of surrounding Pac-Man's body (excluding arms and legs) in chrome, these turn him into solid metal.