SketchUp

3D modelling used to be an advanced technology for which you needed complex and expensive autocad programs. But now the good folk at Google have made it available to the masses. SketchUp is a smart little tool that allows you to easily create 3D drawings. The paid-for Pro version will give you all functionality, but the free version will be quite enough to make a nice drawing of that desk you were going to build.
not open-source, but it is free

download SketchUp