There's a good list halfway down the right hand column on the Apple iMovie page. I don't have any myself (use Final Cut Express) but they usually have demo quicktime movies of effects for you to try. There are loads from transitions to effects to picture on picture, etc. I think eZedia is supposed to be good.
There are also demo versions of plugins you can download from Stupendous Software... I think if you like them you can buy them in packs for around $25. http://www.stupendous-software.com