Copy.sh is an emulator that runs inside your browser and effectively is the Windows 98 OS, unlike some other webapps that merely look and feel like old-school windows. The project took four years to write, and more importantly, it comes with Minesweeper!
There are options to boot from a CD or floppy disc image, which presumably means someone with a little tech savvy could play Tribes or Half Life 1 in a browser! Hell, a good number of games from that era are abandonware at this point. That said, I did try to get Rollercoaster Tycoon running without any success.
The browser works too, sort of. When I tried to see what Gizmodo looked like on Internet Explorer, copy.sh went dead for a good two minutes before showing me this message: