VirtualPC - A PC emulator for Mac from Microsoft. Allows DOS, Windows, and Linux operating systems to be installed within the emulator environment. - http://www.microsoft.com/mac/products/virtualpc/virtualpc.aspx?pid=virtualpc
Mac Emu - Allow Macintosh computers to run old games and applications. Includes message boards, chat, benchmarks, and tools. - http://www.macemu.com/