Check out the GUI prototyping add-on Pencil Project for Firefox.
I just discovered the open-source SVG editor, Inkscape.
And it’s not a moment too soon. I’ve been using Illustrator to edit some extremely complicated SVG graphs. Except that when ever I “save-as” from Illustrator, it can no longer open the resulting file! I’m still using Illustrator 10 which undoubtedly is part of the problem. Forced obsolescence.