Re: Decent Open Source 3d/CAD modelling programs?
Sketchup is great. Until you decide to go for something more high end. At that point it becomes very hard to unlearn the methodology of building things in it. If you just want to "play around" I reccomend it, but if you really want to invest some time and get into some serious modelling, Blender is the better choice. It's free and widely supported and will take you further as it has many more features.
And I say this as someone who does all my modelling in Sketchup.
Baby, you and me were never meant to be, just maybe think of me once in a while...