FreeDOS is a free DOS-compatible operating system.
If you're wondering "so what", Google for something like "DOS games". There's a host of classic software around, most of it entirely free to download. Modern Windows can't run any of this, but FreeDOS makes it available again.
Download options include the "CDROM standard installer", a regular ISO image which you can use to install on a computer, or run in VirtualBox or another VM tool. Other downloads include images you can use from a USB key, or a boot floppy.
Setup takes a little work as you're left at the command prompt, but the website has tutorials on what to do next. You could start by connecting to your network, for instance, or perhaps share existing files or downloads with your VM.
Even though DOS is absolutely ancient, FreeDOS isn't just about the past. Developers are still coming up with helpful extras. The FreeDOS website recently mentioned a brand new project called EtherDFS, "an 'installable filesystem' TSR for DOS that lets you share files between DOS and Linux over a network. It maps a drive from a remote computer (typically Linux-based) to a local drive letter, using raw ethernet frames to communicate."