JPC is a pure Java emulation of an x86 PC with fully virtual peripherals. You can go to their site and run the applet demo, which runs FreeDOS and then lets you execute various classic PC-DOS games such as Lemmings or Prince of Persia. And it supports protected mode, so you can run Windows 95 and --gasp!-- Linux.

Because it's an emulator and not simply a hypervisor, you can run it anywhere in which a Java 5 or higher JVM can run.

Mindblowing.

ps: in the same vein, check out this Browser emulator which simulates the experience of older browsers within your... browser. Right.