GOF, Gates On the Fly, version 2.100 has enhanced verilog RTL debug
capability.
.Verilog RTL browser.
.List the design hierarchy.
.Colorize verilog code. Keywords, comments and nets.
.Double click on net to trace the driver or the net.
.Search function.
.Add line index which can be controlled on/off by setup.
.User can control colors of RTL viewer.
.Interface with GTKwave and Verdi.

Check the following link for detail.
http://www.nandigits.com/rtl_viewer.htm

Nandy 
www.nandigits.com 
Netlist Debug/ECO in GUI mode.