Plants vs. Zombies is a great "tower defense" game for the Mac. It was originally released by PopCap Games for Windows and Mac OS X, and it was then ported to other platforms, including iPhones and iPods. The game can be downloaded from PopCap Games directly or via Steam, for those who prefer that platform. There is a demo available that takes you through the first few levels and teaches you how to play.
The game consists in repelling waves of zombies that want to get to your house by using plants. The premise might sound ridiculous at first, but when you see the "peashooter", you will no doubt change your mind. These plants shoot a variety of different things at zombies to kill them. The peashooter shoots peas at them in a linear way. As you progress through the game, new plants become available and new types of zombies appear. They are stronger each level and they come in greater numbers. The game is a bit slow at first, but it becomes incredibly addicting later on. I have this game on my iPhone and it is a great time killer.
The Mac version looks every bit as good as the other versions. You can run the app in full screen or in windowed mode. There are a few game modes for you to try as well, and they are available in the demo as long as you complete a few levels of the adventure mode.