DraftSight CAD
Free CAD software for PC, Mac, and Linux from Dassault Systems (one of the giants in the CAD industry):
(Free for hobbyists/non-commercial use. This is _not_ free for _commercial_ use.)
http://www.3ds.com/products/draftsight/overview/
I found this software via the following YouTuber: knivesandstuff
https://youtu.be/_lj2PN_ ... AAAAAAANAA

123D App
Free PC-only CAD software from Autodesk (one of the other giants in the CAD industry):
http://www.123dapp.com/

MeshLab
Free open-soruce software for processing 3d polygonal meshes:
(This will not work with non-polygonal file-formats used by many CAD programs. For example, spline and NURBS formats like IGES need to be polygonalized first. But this can do stuff like convert .obj files into .step and .stl.)
http://meshlab.sourceforge.net/

There are also several other packages which are either free, or relatively inexpensive. If you get to try any of them out, let us know what you think. I should warn you that 3d modelling has a fairly huge learning curve. It might be getting better with the new software, but it's still quite a lot to take in.