Draftsight
http://www.3ds.com/products-services/draftsight/overview/
It is free and is pretty much identical to autocad. So close I dont know how they dont get sued. It is from the same company that owns Solidworks and Catia.
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The fastest way I have found to use it is to enter the length/radius of the line/circle that you are drawing on the keyboard. Select the tool you want to use (line tool) start drawing the line from your start point and then just type the length you want (say 3 for 3") and hit enter. It will automatically finish the line at the length you specified, etc.