I am and have been a huge fan of the powerful Oracle (formerly Sun) VirtualBox, but if you use VirtualBox and want it to continue to work well, wait a bit before upgrading to the shiny new version 4. As with most big upgrades, this one took 1 step forward, 2 steps back, and a . . . → Read More: Beware the new VirtualBox 4.0.2 upgrade!