What are some great OS X games? Despite being a "subjective" question, it is a useful one for newcomers to the Apple platform, and serves to aide the lacking gaming community.

What are your favorite OS X games?

Rules:


Limit to one application per answer.
Add a short description for the application
Add a link to the website in the name of the application, if possible (no direct downloads).
Use ## [game name](link) when citing the game.


Only Mac OS X games please, iOS games will be in another question.
Portal


  Portal™ is a new single player game from Valve. Set in the mysterious Aperture Science Laboratories, Portal has been called one of the most innovative new games on the horizon and will offer gamers hours of unique gameplay.