VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product for enterprise as well as home use.
Ranked in these QuestionsQuestion Ranking
Pro Many free images available
Though most are linux flavors at https://virtualboxes.org/
Oracle's virtualbox site has others.
Pro Can do snapshots
Pro Works well and fast for Windows host and Linux guest
Pro Free and open source
Pro A lot of customization
VirtualBox has multiple virtualization options. Each one may be tailored to the guest operating system’s needs. This makes it especially good on virtualizing older systems, such as DOS, obscure distros, etc.
Pro Beginner friendly
A virtual machine can be set up in minutes if a ISO file to load is already available.
Con Minimal CPU customization
You can only enable/disable certain features. You cannot cap the speed of the CPU, which is required to run systems such as Windows 95.
Con Lacks video performance
Virtualbox has a hard cap of 256MB of video memory. This could make newer operating systems run slower.