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.

SlickMotion
A stand-alone application that creates high quality pans from high resolution photos. A must for documentaries or any film using many stills. Retains hi-resolution of imported photos!

Great value
$49.95 as a digital download (CD also available)

System requirements:
iMovie HD 6, iMovie HD 5, iMovie 4
Mac OSX 10.3.9, 10.4 (Tiger), 10.5 (Leopard)
Macintosh with PowerPC G3, G4 or G5 processor
Macs with Intel processor

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