Ars Technica has a nice collection of 7 classic versions of Windows and Mac OS you can run in a browser. It is impressive how earlier operating systems are being emulated in Javascript and these emulations are much more accessible because they run in the browser. I had fun trying an emulation of a Mac Plus running Mac OS 7 (see above). I wonder if we could load the HyperCard version of Perseus in it.
Speaking of in-browser emulators, there are also NES and SuperNES emulators. They have a number of games available for these Flash Emulators that you can play in-browser.