iMovie: Creating Videos On Your Mac is a complete course with more than 30 videos and 4 hours of instruction that will teach you how to use iMovie to edit your clips together into short films. You’ll learn how to use all of the tools in iMovie such as the precision editor, transitions, titles, backgrounds, overlays and much more.
A little-known trick will allow you to run your Mac's screensaver as a desktop background. You can use this to have an animated desktop, display useful information, or just for fun. It could be useful in situations where you have your Mac just playing music in a shared space and want something more interesting being shown than just a static image.