I use ArgoUML for UML and UML->code generation. For pretty much any other type of visualization, I use OmniGraffle, but it's only for OS X. If you're looking for a SVG editor/illustrator, SodiPodi is available for free.
EDIT: I now see that weed's recommendation of Inkscape, is actually a maintained fork of Sodipodi... so you're probably best to go that route for SVG.
....looks like my software is way out of date.