No, Graphviz is truly free--it's open source, under the Common Public License Version 1.0.
And, since no one has described it here yet: aside from making pretty-pictures, the really neat thing about Graphviz is that it is declarative--you describe the nodes in your graph (which can include text, shape, color, etc.), specify the connections and voila--Graphviz does the rest.
Oh, and you can use it programatically too--there are libraries for many languages, so you could, e.g., have a user submit genealogy data and then spit back a pretty family-tree diagram.
