VirtualBox for Mac is a virtualization tool that allows you to create virtual machines for Mac OS X, Linux, or Windows running on Mac OS X. VirtualBox for Mac is not available from the App Store but can be downloaded from most download sites. VirtualBox for Mac is open-source software and has no charges associated with it.
VirtualBox for Mac lets you create a virtual machine on Mac OS X that you can then load Mac OS X, Windows (pretty much any variant) or Linux (and OpenBSD) into. As long as you can allocate a decent amount of RAM to the virtual machine, VirtualBox for Mac will let you run multiple operating systems without issue. To test VirtualBox for Mac we loaded up virtual machines with Windows 7, Windows Server, Mac OS X, and two versions of Linux. On our 8GB RAM MacBook Pro we could run two or three virtual machines at the same time (each allocated 1GB RAM) and switch between them cleanly. Access to system resources such as hard-drive partitions, USB devices, CD player, and so on is mostly clean, although some USB devices may cause issues. Fortunately, as an open-source project VirtualBox for Mac has lots of contributors and advisers on its forums that can help troubleshoot issues.
We've used a few commercial (and expensive) virtualization tools on our MacBook Pro, but after using VirtualBox for Mac we think we'll ditch the commercial products. VirtualBox for Mac lacks nothing the commercial products offer except perhaps a tech support phone line. A very nice product.