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.