VirtualBox

This piece of open source software is worth trying. It is not an application; it does nothing by itself. All it does is to create a virtual computer. Instead of buying a computer, you start a virtual machine. Think at the enviromental impact. You can even start many virtual machines simultaneously.
In my case, I own a Mac but have found some interesting games that require Windows. I have already tried BootCamp, that is perfect, yet I also liked the possibility of keeping my game open while I was working. It made no sense to buy a commercial software to run a free game, but Virtual Box is different, because it is free.
Advantages:

1. Is free


2. Is backed by a big company like Oracle


3. Easy to install


4. Many things work without the need of configuring them, for example when I install an operating system on a virtual machine, it is already connected to the internet

Limitations:

1. I have found a program (Chessmaster) that does not run on the virtual machine. Maybe you will find other programs that won't run.


2. Though the manual says it is possible to share a folder between the host system and the guest system I have not been able of sharing anything.

If I really need to exchange files between my real computer and the virtual machine, I send emails to myself. For this reason, I use the virtual box only for playing. Using it for real work is not practical, and probably it's not even safe. Anyway, for the average Mac user, there is no reason to use Windows for work. Maybe the the contrary is true, that is Windows users might need to install Mac OS on their machines. This is impossible, or at least not permitted.
Oracle is actively working on the VirtualBox and it may become a planetary success in the future.