To make professionally-looking diagrams without getting expensive software you might want to checkout Inkscape, which is a free and open source scalable vector graphics editor available for Windows, Mac OS X, and Linux. Being vector graphics, the actual diagrams, shapes, and objects that you create  are resolution-independent. This is perfect for publishing as you can scale your images any way you like without losing details. Inkscape’s default format is SVG, but you can save to many other formats, including the standard EPS format for inclusion in LaTeX documents. I have tested the software under OS X 10.6 Snow Leopard, under which it runs fine but requires X11.

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