Windows/Mac/Linux (All platforms): Free and open-source virtualisation tool VirtualBox recently released a 2.0 version that includes a few major upgrades. Most importantly, 2.0 supports running 64-bit operating systems, like Windows Vista and certain Linux distributions, on a 64-bit computer. Windows, Mac, and Linux clients have all gotten an upgrade in the looks department, and networking support has been boosted for Macs (improving its standing as a free alternative to Parallels and VMWare). Performance improvements for AMD chips is also baked in, so most users can find a good reason to upgrade. VirtualBox is a free download for Windows, Mac, and Linux systems.