Christmas has come early for users of VirtualBox, a free and open source virtualization solution from Sun Microsystems. Version 2.1 is a huge upgrade to the product as it includes VT-x and AMD-V hardware virtualization support on OS X and full VMDK/VHD support — including snapshots — putting it on par (at least from a hardware perspective) with Parallels and VMware.
This release also includes support for 64-bit guests on 32-bit host operating systems and experimental 3D acceleration. Networking performance has been boosted and there have been many smaller bug-fixes and enhancements.
If you only have occasional need to run Windows, Linux or alternative operating systems but do not want to pay for a full VMware or Parallels license, VirtualBox is a great alternative.
Be sure to check out their Mac-specific forums if you run into any quirks or difficulties. I’ve found the community to be very helpful and responsive and run a few flavors of Linux in VDIs primarily to be able to take them anywhere (VirtualBox is cross-platform). While it does not have as extensive of an integration between the guest and host systems and there is no support for OS X virtualization yet, VirtualBox is a solid product with a promising future.