Makezine recently featured an interesting article on "How To Emulate Rare and Retro Platforms on the Raspberry Pi.
Among others: Commodore Pi is a native Commodore 64 emulator for Raspberry Pi. It boots nearly instantly, can access the full RAM of the Pi and takes advantage of the GPIO, Ethernet, and USB ports.
MAME, the Multiple Arcade Machine Emulator. PiPlay (formerly PiMAME), a pre-built Raspberry Pi operating system made for gaming and emulation. It can emulate many retro consoles including Playstation, Genesis SNES, NES, Gameboy, Atari 2600. PiPlay has a built in FTP server and web-based uploader utility that makes adding game ROMs easy.
The PC XT-compatible emulator 8086tiny is written in C, it can be compiled fairly easily on the Raspberry Pi. The open source distribution includes FreeDOS and the classic game Alley Cat. The emulator is powerful enough to run Windows 3.0 (as pictured above) and even AutoCAD! (Also be sure to check out FastDosBox, which is optimized for DOS gaming on Raspberry Pi.)