Question

I suddenly realized that there is no Altera Quartus or Xilins ISE or ModelSim on Mac OS X.

What do people use to at least simulate VHDL and schematic designs on Macs?

Answer 1

Try GHDL (alternate link: at ghdl.free.fr).

From the site:

GHDL is an open-source simulator for the VHDL language. GHDL allows
you to compile and execute your VHDL code directly in your PC.
Combined with a GUI-based wave viewer and a good VHDL text editor,
GHDL is a very powerful tool for writing, testing and simulating your
VHDL code.

GHDL is based on the very popular GNU compiler GCC and runs on Linux,
Windows and Apple OS X. You can freely download a binary distribution
for your OS or try to compile GHDL on your own machine.