Doulos KnowHow
Designer's guides and models for VHDL and Verilog. SystemC home. Also Perl and Tcl/Tk for hardware designers resources. http://www.doulos.com/knowhow/
Digital Core Design
VHDL, Verilog synthesizable and simulation models. http://www.dcd.pl/
HDL at Wikipedia
Definitions, resources, and links related to hardware description languages. http://en.wikipedia.org/wiki/Hardware_description_language
ABEL Primer
Overview of the ABEL Hardware Description Language. http://www.ee.upenn.edu/rca/software/abel/abel.primer.html
Sim-nML
A high-level processor specification language. it views a processor as a machine that executes a set of instructions. syntax, image, functionality (side-effect), resource-usage and timing of each instruction are described in a hierarchical form. http://www.cse.iitk.ac.in/sim-nml/
ForSyDe
The ForSyDe (Formal System Design) methodology has been developed with the objective to move system design to a higher level of abstraction and to bridge the abstraction gap by transformational design refinement. http://www.ele.kth.se/ForSyDe/
Ruby
Ruby is a notation and design discipline intended for the development of regular integrated circuits and similar hardware and software architectures. http://web.comlab.ox.ac.uk/oucl/work/geraint.jones/ruby/
Titivillus: Software Development
Links to many programming languages, including several HDLs. http://tos.maintree.com/titivillus/softwaredev.asp