From the Things I Never Knew™ department…
You can look up words in the OS X dictionary by tapping with three fingers on a trackpad (in Lion, use a three-finger double-tap). You’ll see something like this:
You can learn about this, and a whole slew of other gestures, on Apple’s Mac Basics: Multi-Touch Gestures page.
I’ve already written about how annoying I find these gestures: Gestures: Hard to use, harder to find. I wish they were easier to discover. The Apple web page is worth checking out to see how many you don’t know.
h/t Rob Griffiths.