If you haven’t heard of GHDL, it is *the* free open-source VHDL simulator out there.
GHDL stand for “G Hardware Description Language” (the G is without meaning). GHDL is mainly implemented in Ada and can be build with different backends: mcode, LLVM and GCC. The different backends provide different performance levels and vary in build complexity. I recommend LLVM since it performs well and is still quite straight forward to build. Building GHDL from latest sources from its github project is probably the best way to go.
Despite its free nature GHDL provides very good support for all major VHDL-LRM releases: VHDL-1987/1993/200X/2008(partial). Unfortunately GHDL is a pure VHDL simulator, so there is no support for Verilog at all. This is understandable as there are already some very good simulators for Verilog out there.