In the Spring trimester of 2011, I took a course in hardware organization at my university. As a side project, I designed and built a CPU in Logisim logic simulator.
The CPU is still a work in progress, but I've made it (and some accompanying materials) available here online under a CC license in case anyone wants to poke around with it. It's incomplete and there's a lot of work yet to do before it'll be fully functional.
There are no instructions, so... good luck!
MIPS CPU Project by Erty Seidohl is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
CPU Version 0.1 - Runs in Logisim 2.7.0 or higher
MIPS Instruction Reference - The subset of MIPS instructions that the CPU should run when complete. NOT guaranteed to be correct! (xls format)
Powerpoint presentation (2010-03-09) (ppt format)