Verilator: Fast, Free Verilog HDL Simulator

Verilator is the fastest free Verilog HDL simulator, and outperforms most commercial simulators. Verilator may not be the best choice if you are expecting a full featured replacement for NC-Verilog, VCS or another commercial Verilog simulator, or a Verilog compiler for a small class project (we recommend Icarus Verilog instead.) Verilator has about the same performance as the leading commercial Verilog simulators including Carbon Design Systems Carbonator, Modelsim, Cadence NC-Verilog, Synopsys VCS, VTOC, and Pragmatic CVer/CVC.

Source: www.veripool.org