The one your employer provides for you. The high end commercial programs (such as Autocad and Solidworks) are very expensive and you probably don't want to buy them yourself.

2D - Boxford 2D Design



3D - Google Sketchup

2D - Microsoft Paint





3D - Minecraft

2d-draftsight (free)





3d-sketchup (free)

I use a program called "Design CAD" its inexpensive, and works very well.



http://www.imsidesign.com/Products/Desig...