In a bid to justify buying a 3D printer, I've been looking into 3D modelling software.

I have some experience with 3DS MAX and SolidWorks but have never been good at modelling organic things.
I have found a cool, intuitive piece of software for 3D modelling called Silo. It's only 20-25MB and cost $109 but has a 30 day trial.
There is a good tutorial on YouTube (see below) and although I'm not as good as this guy, I have created a robot head on my first attempt with some success. Still learning the controls.

Blender is worth you looking at too and is free.  I use it sometimes, eg :

http://aidreams.co.uk/forum/index.php?topic=3591.0