Ponoko.com recently shared this great tutorial on Inkscape. I regularly use Inkscape to design projects for the laser cutter and vinyl cutter. It's Open Source and free for Windows, Linux, and Mac.
There are plenty of tutorials online for this tool however often the information isn't as relevant to designs for the laser cutter or vinyl cutter. You can make a great looking image that suffers from a lot of problems when you try and cut it out. Read the article to learn about those particulars
With all design software there is a learning curve. When adding CNC to the mix you get additional design challenges and this article will help you understand and reduce the added learning curve.