VMware allows you to run one or more virtual machines at the same time on one computer. You can download VMWare Player for free.

Installing VMWare ESX server

After installing VMWare ESX server, set in /etc/ssh/sshd_config

PermitRootLogin yes

TroubleShooting

If you get a pop-up saying "Failed to initialize mouse-keyboard-screen control", you have probably tried to start your virtual machine in an nx session. Better use a vnc session.