VirtualBox is available for free with source code available here. A "VirtualBox Extension Pack" that includes virtual USB 2.0, VirtualBox RDP & PXE boot support is available only under a commercial license. VirtualBox is available on Windows, Linux, OSX and Solaris.
The pro version which has snapshot support is ridiculously overpriced. What's worse, major releases require a brand new license, but generally don't add much more functionality.
Too often you get "vm is used by another user" or "can't revert to snapshot" for whatever reasons without simple solutions. In most cases, it just throws knowledge base links to you. Not even clickable. Way overpriced.
Parallels has an easy to learn and use UI. All important features are wizard based. It requires only a few clicks for the wizard to download and set up the latest Chrome OS, Windows or Linux build.
Parallels has a "Coherence View" mode that allows running Windows applications as if they were native OS X apps. In this mode Windows is virtualized in background and Windows features like Jump Lists, Taskbar, Start Menu, etc can be embedded directly within OS X.
Chances are that the same version of Parallels won't work with different OS X releases, requiring a separate purchase of Parallels for each OS X version.