Classic Mac OS and dozens of apps can now be run in a browser window
The Internet Archive is a great resource if you're looking to play with older PC apps and operating systems—thanks to a JavaScript port of DOSBox, you can run stuff like Mario Teaches Typing and Windows For Workgroups 3.11 right in your browser, giving you a quick and easy way to get some idea of what it was like to use a computer 20 or 25 years ago.
Now, the Internet Archive has some retro computing offerings from the other side of the great Mac/PC divide. Using a version of the PCE PC Emulator that has been ported to JavaScript, people interested in the Mac's early years can run System 6, System 7, and dozens of old apps, including MacWrite and Microsoft Basic using their browsers.