"Pac Man" can be played for free online on many different websites. Freepacman.org has a good version of the game that is a remake of the recognizable classic gameplay.
"Pac Man" is a classic game that is relatively easy for game developers to make, so you can find a lot of different versions of it online. "Ms. Pac Man" is a classic version of "Pac Man" that is a bit more complex and more difficult than the original. You can play a remake of it at mspacman1.com. Some versions expand on the original game with their own features, like worldsbiggestpacman.com, which has players playing in a massive maze made up of stitched together user-created mazes.