Speaker Simulators

Speaker simulators are a bit like equalizers in that they adjust the frequency response of the signal. They do it to emulate the response a guitar amp would give you. Some are more refined and add other things to the signal.

Links

Condor Cab Sim - 4x10 cabinet simulator by Run Off Grove.
Speaker Simulator - Tonepads speaker simulator.
Multicab SIM - Semi low parts count.