The other commentator mentioned Verilator (which is indispensable in larger designs) but you may also want to grab Icarus Verilog too. It's a FOSS simulator and, unlike Verilator, is 2-bit and so it handles X ("don't care") and Z ("high impedance") signals. It's ridiculously slow compared to Verilator but the greater fidelity can be valuable depending on what you're trying to do.