You can't make animation with iMovie.. I use iMovie only for adding subtitle.
For mac, there is Motion. You can do anything basically, it has layer, keyframe, and camera, similar to After Effect. It has huge variety of preloaded motion & effect too.
If I'm not wrong it's around $45, It's cheap if compared to After Effect.
and.. I heard that a lot of Niconico users use Aviutl. It's free, and I found out the english patches here: http://www.htasoft.com/aviutl/aviutl.htm
* Aviutl is for PC though..