I don't know actually, if AC3D is the original name or short form... XD
It's not free, but it's not as expensive as 3D Studio Max or other advanced 3D programs, and has a trial. I primarily use it because I make things for Second Life, and AC3d does what I need in order to comfortably create and export my models to the game.

Sketch-up.... I believe that's aimed more towards architecture, no? I never really tried it, but it seems like something that's more for building homes and straight geometric figures, as opposed to organic shapes like faces.

If you want something that can really be like clay-modeling, I hear zBrush is like that, and good for very detailed/organic models. I never tried it either, but my friend swears by it. I don't know if it's free or not.

I know Blender is completely free, but that program really pisses me off. I hate the controls of it, and the interface is so complicated and confusing. @_@

Maya you can get free if you have a student ID. It's one of the top 3D modeling programs, along with 3D Studio Max.