VirtualBox is an open source project managed by Oracle. This basic virtualization application allows you to run Windows, Linux, and other operating systems on a Mac, without ever having to reboot.
VirtualBox has long been a favorite of those who need to run a non-Mac OS, but who don't need some of the high-end features found in commercial virtualization applications, such as fast graphics performance.
Setting up VirtualBox may require a bit more technical knowledge than other virtualization apps, but its 300+ pages of documentation can answer just about any setup question you may have.
VirtualBox 4.x is free for personal use.
See other software choices from Tom's Mac Software Picks.