Here are some free program's that Quark agree's on that you wont go wrong with.
Blender -5 Latinum Bars 3D Modeling progrem
Sketch Up - 3 Latinum Bars. 3D Modeling program (only because the better features of the program require you to purchase the program which is around $400.00)
My Paint 0.9 - 5 Latinum Bars. Paint program with many different brush strokes available to create meshes with. My Paint is also great for rendering background scene's. Great for creating realistic images of nature and biological entities.
Gimp - 4 Latinum Bars. Vector drawing program coupled with low function paint capabilties.
If any one can add onto this list please do and continue to render on. The more starship modeler's we have the more the future will become real. Don't get frustrated when using these programs as all of them have great tutorial's included.