Sketchup's been out for quite a while. It's ok as a modeler, I hear, but the problem is that it creates models in a proprietary format that only Sketchup (the free one) can use.
I use something called Wings 3d, which is free, and exports in several different formats, which can be used in other art programs like Poser, Vue, Bryce, etc. It's called 'box modeling', where you take a primitive shape and manipulate it in all sorts of ways.
The one I found kids really like is a Japanese program called Doga L-1. You get robot and spaceship parts, which you attach to each other, size, scale, rotate, whatever you like. L2 and L3 cost money, but it's the next best thing to modeling if you don't know how...

Other free modelers include
Truespace (some versions)
Blender (open source; a little confusing with all the separate menus..

Amapi (some older versions)
Anim8tor ( a single file, basic, but people have used it)
I haven't made anything much more complicated than a 3d model of a game level of Zork (trust me, the game's likely older than most of the people here..

the 'Relativity' building by Escher, and any number of spaceships...

ive got blender, i really want to learn how to use it but there's so much you gotta learn before you even attempt to make something with it i just get put off it