HDLs can be synthesized into ASICs, so any open hardware implementations are important as they can be incorporated into real ICs.

http://www.opensparc.net/ Several commercial SPARC CPUs. Seems to be sponsored by Sun Microsystems.

http://opencores.org/ Large collection of open source HDL by various contributors.

http://tams.informatik.uni-hamburg.de/research/vlsi/vhdl/ http://tams-www.informatik.uni-hamburg.de/vhdl/vhdl.html Small, but has a few source files and some links.