In this video, Macworld shows you how you can easily install and run OpenSolaris on your Mac computer using a free virtualization software from Sun – VirtualBox.
The tutorial shows how to create a Solaris virtual machine but you may use the same procedure to run Linux (like Ubuntu) or even Windows on your Mac. Other software that let you run multiple operating systems on a Mac include Parallels and VMware Fusion.
VirtualBox is an open-source virtualization software for Mac OS X from Sun Microsystems and is available for free.