F-CPU, mainly dedicated to pure SIMD superpipelined 64-bit RISC CPU, and the sources. Open Source, GPL.

For advanced hobbyists: share free microprocessor and DSP IP cores written in Verilog or VHDL.

32-bit, 2-way superscalar RISC processor, designed in a HDL. Source downloads. This one is actually working.

An open implementation of IA-32 architecture which is also called x86. Includes documentation, screen shots and a forum.

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.