Inkscape - A Free Vector Graphics Editor

If you need something capable of drawing vector graphics diagrams and can't afford the likes of Adobe Illustrator, Freehand or CorelDraw, then take a look at this open-source solution…

Inkscape - This is a vector graphics editor using the W3C standard Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as JPEG, PNG, TIFF, and others and exports PNG as well as multiple vector-based formats.

Like a lot of other open-source applications, the Mac OS X version requires the X11 windowing system so it won't win any prizes for interface usability but it does work. The developer's main goal was to create a powerful and convenient drawing tool fully compliant with XML, SVG, and CSS standards and it does that pretty well. On top of that, the web site has a pretty good set of documentation, galleries and tools.