Keen to get into 3D Modelling, check out Google sketchup for a crash course. Also not bad if you want to quickly mock something up. One of the easiest to learn and it doesn't take long to actually produce reasonable mockup. Not to mention its free, unless you want the pro version which is about $500.

http://www.google.com/sketchup/


If your really keen then you may want something with a little more functionality, enter blender.Geared towards CGI and 3d modelling with animation. Blender is also freeware.


http://www.blender.org