cMIPS is a model in VHDL that mimics the pipeline design described in Patterson & Hennessy's book (Computer Organisation and Desing) and is an almost complete implementation of the MIPS32r2 instruction set. There are a few opcodes missing, mostly to do with the Control Processor (COP0).
The processor executes code generated by the GCC toolset without changes. An stdlib lookalike is in the works, as well as a port of embedded Linux.
The floating point instructions are not implemented, and a TLB plus assorted control registers will be added soon (as of oct2013).
The project is hosted at Sourceforge. A more complete description can be found at docs/cMIPS.pdf, in the repository.