Icarus Verilog - Free, open source Verilog compiler and simulator. One of the most comprehensive, free Verilog simulators that I have used and is still in active development.
RealMacSoftware - Producer of the RapidWeaver product that was used to create this website.
gEDA - Suite of EDA applications for simulation, debug and backend work.
GtkWave - Free, open source VCD dumpfile waveform viewer.
Doxygen - Free, open source documentation system for C, C++, and many other languages.