The two best virtual machine player are VMPlayer by VMWare and VirualBox by Sun.
Both applications are free and can be downloaded from their websites.
How do they compare?
Well, both applications more or less perform neck and neck. But I wound say vmplayer performs slightly faster in terms of speed.
Out of the box, virtualbox has a snapshot feature where you can do a snapshot of our stable version of your preferred OS; and whatever happens to your OS i.e corruption or virus attack, you can always roll back to your previous snapshot. VMware player lacks this feature but if you upgrade to VM workstation, snapshot will be available.
VMware player has a cool feature called ‘Unity’ where your navi machine is visually merged with your virtual OS, but behind the scene they are both running independently.
Both applications can run several OS at the same time depending on your hardware. Each installed OS, can be allocated memory, hard drive space etc.
Give it a try!