CAD
Plenty of CAD monkeys here to give you advice.
If you want something free to dabble in, then go for ProgeCAD. http://www.progecad.com/. I downloaded it and gave it a try many months ago just for kicks. It has the look and feel of AutoCAD to the point where I'm surprised they haven't been sued by Autodesk. Hopefully it also has a decent Help file to get you started.
Another one out there that looks similar is Cadopia. It may be free, but I'm not entirely sure, it also has the same look and feel of AutoCAD from looking at the screenshots. http://www.cadopia.com/
Steve