Run Windows 95 to XP, Mac OS 8.6 to 10.4 in your browser, sort of

Complete desktops contain all operating system components as well as Internet Explorer and Outlook Express. Where possible, I have tried to include built in file transfer programs (Web Publishing Wizard, Web Folders), useful system tools (System File Checker, System Restore) and certain wizards (Network Setup Wizard, Internet Connection Wizard). As a result, some of the desktops are quite large and can take some time to load.
↫ VirtualDesktop.org

These are easily loaded virtual machines [correction: they’re not virtual machines – they’re more like fancy interactive screenshot-like things? Still interesting, but they’re not virtual machine like I thought. Apologies!] inside your browser, for various versions of Windows and macOS. There’s more and more of these websites now, and while I don’t use them for anything, they’re still quite handy in a pinch. And let’s face it – it’s still kind of magical to see entire operating systems running inside a browser.

The website also has several virtual machines without applications, and application-specific virtual machines, too, focused on browsers and mail clients.