GHDL is the leading open source VHDL simulator.
*** Now on github.com/tgingold/ghdl ***
We have binary distributions for Debian Linux, Mac OSX and Windows. On other systems, getting GHDL from here means downloading the current source package and building GHDL from source. Alternatively you can get the latest source version (warning : occasionally unstable!) by pulling a snapshot from the git repository.