Virtualbox by Oracle is definitely the best choice when you need a free solution for your virtualization needs.
Using it is pretty straightforward, but recently I had an issue with the screen resolution running Linux (Kubuntu 13.04) on a Windows 8 host.
What I was trying to do was running Linux on my HP Pavilion 14-b107sa Ultrabook (14″ screen, 1366×768) in a Virtualbox virtual machine hosted in Windows 8.
Everything was working fine except the resolution of the virtual machine which was locked to 1024×768.
After few Google searches I found out what I needed to do was installing Linux guest additions using the command:
sudo apt-get install virtualbox-guest-dkms
which basically installs a package which comes with all the dependencies needed to run Linux properly in a Virtualbox virtual machine.
After rebooting the virtual machine everything worked as expected, both in windowed and fullscreen modes.
For more info about Virtualbox guest additions you can check chapter 4 of the Virtualbox user manual.