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).