What is "gtkwave"

GTKWave is a fully featured GTK+ based waveform viewer which reads FST, LXT, LXT2, VZT, and GHW files as well as standard Verilog VCD/EVCD files and allows their viewing. GTKWave is developed for Linux, with ports for various other operating systems including Microsoft Windows (either natively as a Win32 application or via Cygwin), and Mac OS X targeting either X11 or Quartz. GTKWave is one of the applications affiliated with the open-source gEDA Project.