There's a lot of CPUs around in VHDL or Verilog, e.g.

* http://opensource.zylin.com/zpu.htm

* http://www.altera.com/devices/processor/nios2/ni2-index.html

You could use ghdl , http://ghdl.free.fr/, and supporting tools for simulation. Most FPGA vendors have a free version of their tools as well.