Ranked in these QuestionsQuestion Ranking
Pro Free, mostly open source and cross-platform
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.
Pro Can do snapshots
VirtualBox is capable of taking whole-system snapshots and writing them to a backing file.
Pro Works with Vagrant
Works with Vagrant, an awesome tool for managing and deploying virtual machine images.
VirtualBox 5.1.22 will not work with windows 10
Con Inferior performance
Apart from second-class performance in virtualization, there are some visual glitches with Windows 8.1 on Mavericks.
Con Confusing interface and lacking usability features
VirtualBox might be a bit confusing for beginners - it requires an understanding of a lot more technical details than other VMs.