If you have used Ubuntu before, you have probably heard about running a Windows application in something called WINE. Wine is a nice application, but it is not supported. The officially supported and commercially funded version of this application is called CrossOver.
You can get CrossOver for both Linux and Mac OS, but for the most part it is more popular in Linux distributions. The application usually costs around $25, but today only it is free. Another free offer has been done around 4 years ago.
CrossOver/Wine allows many games to run on Linux/Mac OS, such as Portal 2, World of Warcraft, etc. In order to get CrossOver for free, all you need is a web browser, an an email address. All you have to do is type in your name, your email address, and you're done. You do have to confirm your email by typing it again in the box, but it is pretty standard to do that nowadays.