VMware - a commercial product which virtualizes an x86 PC, allowing one to natively run an operating system in a window on x86 hardware.
WINE - a free reimplementation of the Windows libraries, *nixified. This library is no different from your libc, so to speak. By this I mean absolutely no emulation or virtualization is done. Can load Windows .exe's on x86 platforms.
bochs - a free 486 emulator (NOT virtualization!) Code is interpreted, not run natively.
plex86 - a free reimplementation of VMware based on bochs. Essentially, bochs with the emulation engine replaced with virtualization.