Sun Microsystems has released the code for its UltraSPARC T2 chip, which also goes by the code name Niagara 2. The source code can be downloaded from OpenSPARC.net and you are required to register to get it.

While it is unlikely that anyone will want to build their own UltraSPARC chip, earlier open sourcing of the T1 chip has resulted in some interesting spin-offs for this technology. The most notable is the Simple RISC S1.