Matte-tastic
Superimpose logos or any matte onto moving video.

VidMix
Finally,blue screen technolgy comes to iMovie. This plug-in lets you merge two video sources using color key, brightness orcutout.

Picture in Picture
Two video sources with one in a positionable and sizeable internal box.

Split Screen
Two video sources split and combined or one or both squeezed.

Great value
$29.95 as a digital download (CD also available)

System requirements:
iMovie HD 6
Mac OSX 10.3.9 - 10.6 (Snow Leopard), 10.7-10.8

iMovie '08 does not support any plug-ins. To use Slick, get iMovie HD 6 for free. See more info here.