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.
Compared to other VMs, Parallels has the best gaming performance. Still, a good framerate can only be achieved with games that have very low resource requirements. A much better option for resource intensive games is dual booting Windows (via Boot Camp).
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.
Parallels upgrades are very expensive. Even if I bough several versions of Parallels I will switch back to free VirtualBox as they even "invented" the Pro version - mainly doubled the price you have to pay each year to have the software updated.
Vertuu uses whitespace and subtle icons extensible making it feel like a very light and modern app. The layout list of the users VMs is well designed with 3 well recognised icons on the left, and name and status on the left.