Most good quality games out there today cost money and run on either Windows or one of the game consoles. There are very few Mac-only video games, and out of those, I doubt that most would be free. If you are looking for online, Flash-based games, many casual game sites should work. But why game on a computer anyway? I've found dedicated gaming hardware to be more enjoyable and entertaining.
Or, if you chose not to read my above comments, go to your Applications Folder and select "Chess". That's just a "somewhat" popular game that is built into OS X. Why, you can even play it with voice commands against the computer! How's that for a good game?
If that doesn't float your boat, then you might as well grab a copy of Halo and play on an Xbox. Talk about classic. That, or you can play Minesweeper on a PC. Enjoy.