Raspberry Pi Emu (or RaspiEmu) is a Raspberry Pi emulator, capable of running almost any Raspberry Pi (ARM architecture) operating system.
The Raspberry Pi is a credit-card-sized computer. The operating system on most Raspberry Pi's is "Raspbian", which cannot run on i386 desktops or laptops. This is meant for developers, or end consumers who do not wish to fiddle with the command-line, do not have a Raspberry Pi available or are too lazy to use a physical Raspberry Pi.
Note: While the software is classified as free, it is actually donationware. Please consider making a donation to help support development.