Dia is an excellent diagram creation tool most similar to Visio for Windows. It runs natively on linux using GTK+ and can (with some finagling) be installed on OSX using Fink. Things are easier on windows with the Dia for Windows binary installer. You’ll also need to install GTK+ for windows but that’s trivial. Instructions and links are on the Dia for Windows site. If you’ve needed a diagramming tool on Windows but can’t swing the cost of a Visio license Dia is an excellent alternative.