QCad is pretty cool. I used it to design the sides of my cab, and since it saves natively in Autocad's .dxf file format, a guy at work was able to trans-mogrify it into code that runs on a CNC router that I run! I got all my side panels cut in 10 minutes. Very highly recommend it. The demo is fully functional, and after the 10 min time limit expires, it lets you save your work, so you can reload the program. The full version is only about $30 and can be instantly downloaded after you buy it(the link stays active for a year). You also can get tons of insertible items like electronic components and such for free!
http://www.ribbonsoft.com/qcad.htmlA scroll wheel mouse is highly recommended, BTW.
Hope this helps

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