Run another operating system

Hypervisors for trying an OS in a window, or running a rack of them on one box.

VirtualBox
The free desktop hypervisor most people start with: run Windows, Linux or BSD in a window on the machine you already have.
Free Open sourceWindows macOS LinuxVirtual machines
QEMU
The machine emulator and virtualiser that most of the Linux virtualisation stack is built on.
Free Open sourceWindows macOS Linux Command lineVirtual machines
Proxmox VE
A bare-metal server platform that runs KVM virtual machines and LXC containers from one web interface.
Freemium Open sourceLinux Self-hostedVirtual machines

VirtualBox