With the move to the Intel architecture, Macs finally had the advantage of using virtualized hardware that was not available in a decent form when it was PPC (the only real contender then was Microsoft's VirtualPC which was less than stellar). Today, the top players for virtualization on OS X are VMware with its Fusion product, Parallels with its desktop offering, and Oracle's free VM VirtualBox software, which came with the acquisition of Sun Microsystems.
As of this writing, the current version of VirtualBox is 3.2.6. The question I had to answer was how it measures up to the current heavy-weights: Vmware Fusion and Parallels Desktop.
As with Parallels and VMware, VirtualBox has community-provided, precreated virtual machine images that can be downloaded and used. A lot of these are appliances to perform a specific task: run a wiki or some specific server software. All of them are built on open source operating systems like Linux or FreeBSD. The Free VirtualBox Images web site is one site that will let you get up and running quickly with a distribution, saving some hassle in downloading ISOs and installing.
To use these VirtualBox images, you need to create a new virtual machine and select the image as the drive image. To do this, click the New button and go through the creation wizard. When it comes time to select the hard drive, or create a new one, select the .vdi image file you downloaded (and uncompressed). Once this is done, the details will show up.
By default, VirtualBox creates the network interface of type NAT, so it will use the host system as a firewall of sorts. If the virtual machine needs to talk bi-directional to the host system, use the Host-only Adapter, and if you want it to be reached by any system (i.e., a physical machine on the network), select the Bridged Adapter. Then click the Start button to start the virtual machine and your new appliance will boot.
VirtualBox is pretty decent, considering the price tag. It definitely has some rough edges -- I'm not sure how well it works on Linux and whether it is a match for KVM there, but on OS X if you want more polish, you'll need to spend a few dollars for VMware Fusion or Parallels to get it.
Having said that, if your budget doesn't allow for it, VirtualBox works quite well and perhaps if the guest is Windows it would have even better support (I found the Linux guest support to be much less functional than on VMware Fusion, but do not have a Windows license to use to see which offers better Windows support).
For the price, it works well enough. Perhaps Oracle will do with VirtualBox what Sun didn't, and turn it into a better product that can compete with the current standards for virtualization on the Mac.
Vincent Danen works on the Red Hat Security Response Team and lives in Canada. He has been writing about and developing on Linux for over 10 years and is a veteran Mac user.