(Free Software) Open Source Inkscape: vector program
Manipulate Vector Images with Open Source Inkscape [via lifehacker.com]
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.
I haven’t tried it myself, but it looks like a great place to start. You could also use the basics of my pen tool tutorial with it, I wrote with it Adobe Illustrator and Photoshop in mind, but vector graphics generally work about the same.
