The higher end stuff Lightwave, 3DS Max, Cinema 4D etc... generally have steep learning curves and are pricy.
Then there is google sketch up which has never really floated my boat.
A very powerful one, commercial grade that is multi platform and open source is Blender. I never dug it's UI though.
The question is how much can you tell us about the object, meaning is it a more organic shape or multi part mechanical?
There are different types of 3d modeling apps that are better suited for one vs the other.
Some use basic shape, boolean and spline based modeling while others work more like modeling clay.
I've heard good things about Sculptris but have not had a chance to take it for a drive.
Pixologic :: Sculptris Multi platform and free.