Some of these are real operating systems, some of them are not. They are all useable in the browser.

Windows 93 - Miss Windows 95? This one is scarily similar, but…in the browser. It has a lot of “programs” that are fun to play around with.

Windows 96 - This one was inspired by Windows93.net, but it feels(to me) more like an actual OS. There are fewer games than Windows 93 though.

Cyber Space and time - This one is another “fake operating system” like Windows 96/93. However it looks like someone ate a bunch of 1990s internet and threw up on the screen. Some neat stuff here and definitely worth looking at, but not as good as the others.

WinXP - This is not actually XP. It is just a website made to look like it and act like it. Still, its kind of neat.

Win32.run - This one is like the one before…its not real. However it FEELS real and had me fooled until I read the reddit post about it.

Infinite Mac - This allows you to run any version of classic MacOS before OSX. Unlike the previous listings, these are real deal operating systems!

OS.JS - This is more of a platform for making your own virtual desktop than an actual virtual Desktop, but they have a live demonstration thats fun to play with!

daedalOS - This virtual OS is the personal site of Dustin Brett. Has an app for his blog, his videos, his music, etc. He linked it to me over HackerNews and while its not the most customizable, I thought it was pretty cool.

Windows Really Good Edition - This is more of a joke, making fun of Windows ME. However, its the oldest “virtual OS” that I could find, having been developed in 2001. It runs in Flash and runs with Newgrounds Flash compatibility layer, Ruffle. One quick warning, if you have epilepsy clicking paint.exe or Windows Media Player is a bad idea.

Krazam.tv - This one is pretty much just a fancy landing page for a youtuber called Krazam. It has some neat features. I like the “safe space” program that gives affirmations such as “You contribute meaningfully to incident management discussions and casual conversations.” or “DevOps is a meaningful term. You understand DevOps because you use it every day.” Pretty humorous stuff! There aren’t any games unfortunately and it can only handle having one window open at a time. Still, its a lot of fun to toy around with for a few minutes.