Graph Visualization Software is a set of open source utilities by AT&T to create and lay out graphs and is used extensively for charting. It takes input in text mode and display them in presentable formats like images and svg for webs.

It has the following features:
dot language: for giving inputs
graphical web interface
lefty: to manipulate graphs with mouse
dotty : to edit graphs
WebDot:render graphs in HTML
It supports windows and Linux. There's a separate version for Mac OS X with all features.It is quite good software design , networking database , web designs and some physics related application.s
Comparison are bound to be with visio. So to make one point clear it is not that good. It caters different genre. It is included by default in SL5.If you are looking for visio alternative then use kivio its quite great.