Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format.
Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care has been taken designing the streamlined interface.
It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more.
Free for Mac OS X Tiger and Leopard.