Fluid is a screen saver that gives you, well fluid visual effects. It has a whole bunch of preset effects and a mixing desk to mix up your own. Works okay on my PowerBook but ideally needs something more powerful! Mac Mini owners need not apply.
CoverFlow gives you an alternative interface to your music by presenting you with a gorgeously rendered user interface of album art.
USB Overdrive lets you enable and assign all those extra buttons on your mouse without having to purchase Apple’s rather overpriced Mighty Mouse. You can get the superior laser-tracking Logitech MX-1000 for the same price if you shop around.