Solution 1:

Have you tried Blender?

Solution 2:

My personal favourite is Maxon Cinema 4D. It has a nice learning curve, is fairly respected in the industry and does what you require very well (simple objects). It has a slightly Adobe-like feel to it. I found that for complex organic modelling you'd need to start looking at Maya or 3DS Max (PC on Bootcamp).

However, these options cost quite a lot - though C4D is less than many applications and is modular.

A cheaper ($99) option is Cheetah, available through the app store. Basic but nice.

As Nix said, Blender is a fantastic, free option. I just found the learning curve is a little more tricky than C4D.

Solution 3:

AutoDesk makes a Mac version of Maya. I'm partial to Maya as I learned 3D on it. If you're a student, you can get an educational discount, which is how I got it about 4 years ago. http://www.autodesk.com/products/autodesk-maya/free-trial

As others mentioned, there is also Blender which is free. I tried it but it there is a bit of a learning curve as matt_w mentioned.

Solution 4:

Here's a pretty big list, with, presumably, all the info you might need. A description, the price, the requirements, and links to the Downloads page.