Virtualization is a pretty awesome part of modern computing. Maybe you want test out some new software without committing to it. Maybe you want to run Linux and Windows and Mac OS X… Or is it macOS now… Whatever. If that’s what you want, then virtualization is for you.
There are some pretty good paid options out there. Parallels and VMWare offer some solid software that isn’t too difficult to use. But if you happen to be tech savvy, and you don’t feel like spending your hard earned bucks on virtualization software, there are some free options out there that are quite good.
Some of these are free as in they don’t cost anything and some are free as in open source (and also don’t cost anything).
VirtualBox is a really good option. It runs on Linux, Windows, and Mac. It isn’t all open source, but parts of it are, and it is free. Read more about VirtualBox here.
QEMU is completely open source. Probably best for virtualizing Linux inside of Linux, although it can do other operating systems. See here for more on QEMU.