ProcessorSim MIPS -- A Visual MIPS R2000 Processor Simulator

http://jamesgart.com/procsim/

ProcessorSim allows you to view a simulation of a processor's internal circuits (components and buses) executing a piece of assembly code, as an animation. It is mainly used for learning how the processor works, and so is useful for undergraduates in Computer Science, or anyone who is interested in how CPUs work.

You can input any assembly program from the selected assembly instruction set, and the simulation will animate the instruction being fetched, decoded and executed by the various components.