Used to be addicted to this on Windows, and it's next step up The Games Factory. It's not as easy as they first make it out to be, but once you get it down, it's incredibly powerful and a lot of fun.

Mind you, I used a much more powerful system, at least a 333mhz PC. Been looking for the Mac version for a long time but have been unable to find it. (not that it would be of incredible use now anyway)