GTKWave is a digital waveform viewer useful for examining the output of various digital simulators and an analysis tool used to perform debugging on Verilog or VHDL simulation models. With the exception of interactive VCD viewing, it is not intended to be run interactively with simulation, but instead relies on a post-mortem approach through the use of dumpfiles. Various dumpfile formats are supported, e.g.;

GTKWave has been developed to perform debug tasks on large systems on a chip and has been used in this capacity as an offline replacement for third-party debug tools. For Verilog, GTKWave allows users to debug simulation results at both the net level by providing a bird's eye view of multiple signal values over varying periods of time and also at the RTL level through annotation of signal values back into the RTL for a given timestep. Terms of license are GNU GPL. GTKWave is separately developed/maintained tools but associated with gEDA suite.