Crossover (previously known as Crossover Office) is bunch of applications that allows you to run native Windows applications on Linux, Mac OS X and Solaris without need of purchasing Microsoft Windows. Really great bunch of tools that allow you to run applications like Microsoft Office or Dreamweaver on Linux easily without hasle of setting up Wine.
Today guys from CodeWeavers have decided to make CrossOver FREE, you can get registration key and it will act in a same way like you purchased application. But even more interesting than giving it for free is reason why they are doing that.
Read more info here :)