All platforms: Inkscape is an open source vector editing program. If you are looking for a free alternative to the Adobe Illustrator, Inkscape has a host of features that make it worth taking for a spin.
Inkscape can be run on Windows 2000/2003/XP, Mac OSX, and Linux. Skilled users of Adobe Illustrator or Corel Draw may find more advanced features missing, but I found in testingâ€” as a casual Illustrator userâ€” that all the features I regularly rely on for vector editing were present and easy to use. Common functions like shape and text tools, drawing, linked images, clones, layering, path markers, etc. are well marked and easy to apply. Inkscape maintains a gallery at Deviant Art to show off work submitted by users, worth checking out if you'd like a quick peek at the capabilities of Inkscape. For more free image editing goodness, check out GIMP. Inkscape is open-source, with installation packages available for Windows, Mac OSX, and Linux.