Cedega emulates a Windows environment for games to be installed and played. Cedega is not free, and uses a lot of your graphic card, but can play many Windows games right out of the box for you to still be able to purchase games for Windows.
Here is a list of games that run (and how well they run) in Cedega. Each game has a link to a wiki which explains best version to run on, popularity, playability, and a small how to on getting the game up and running. (Note: You do not need to subscribe to view the Game Database)
Wine, like Cedega, emulates a Windows environment. However, Wine is free. Wine can play many games, but it is still in heavy development and can not play all games. Speculation however, is that when finished, Wine will be able to play many more games than Cedega will ever be able to.
CrossOver allows you to run many popular office productivity software applications and games, such as Microsoft Office, Lotus Notes, Microsoft Project and Visio, graphics applications like Macromedia Dreamweaver MX, Flash MX, and Adobe Photoshop, games such as World of Warcraft and Half-Life 2, and many more. Not only that, but CrossOver Linux also allows you to use many Windows Web browser plugins, such as QuickTime and Shockwave, directly on your Linux browser. No Windows Operating System license required; CrossOver is a complete replacement for your Windows OS as far as your applications are concerned.
Here is a list of games (including applications) that run in Crossover. It is focussed heavier on applications however.