Open Source

CMOSexod.com
For advanced hobbyists: share free microprocessor and DSP IP cores written in Verilog or VHDL.
http://www.cmosexod.com/

E-Risc
Project to design and implement scalable RISC microprocessor for embedded applications. All architecture and source code to be released under GNU General Public License, or a slightly modified version that includes hardware, not only software.
http://www.geocities.com/SiliconValley/Chip/5014/

The Freedom CPU
F-CPU, mainly dedicated to pure SIMD superpipelined 64-bit RISC CPU, and the sources. Open Source, GPL.
http://f-cpu.seul.org/

STM
32-bit, 2-way superscalar RISC processor, designed in a HDL. Source downloads. This one is actually working.
http://www.asahi-net.or.jp/~uf8e-itu/

Sun Community Source Licensing (SCSL): picoJava
A core is the central component of a processor or controller chip. Chips using picoJava core are well suited for consumer electronic products running Java programs. Sun Microsytems.
http://www.sun.com/software/communitysource/processors/picojava.xml

Open Source Soft Microprocessors
Free licensing of soft microprocessors lets designers easily modify FPGAs and gives greater visibility into architectures; explains implications of associated licensing models. Dr. Dobb's. <small>(March 7, 2007)</small>
http://www.ddj.com/linux-open-source/197800817/