"Bochs" is an x86 platform simulator. The original simulator interpreted a 486 subset of the IA32 ISA. More recent additions have added Pentium instructions and SMP capabilities. The Plex86 system uses the same devices as the Bochs system, but uses dynamic code translation to build a faster emulator (i.e. like vmware).
The bochs site at SourceForge -- appears moribund
Bochs system emulator
Plex86 system virtualization documentation