I agree with 3Dmonkey. I would like to add a link in the list for Tinkercad. I have used Wings3D, Blender, Sketchup, Milkshape, 3D Studio Max, etc. For quick and easy, or just importing and modifying an STL file, Tinkercad is by far the easiest program I've ever seen.

https://tinkercad.com/