1. Open an application where you must scroll horizontally to see information — like an image file in Fireworks, Photoshop or Illustrator that's been zoomed large.
2. Hold the shift key and use your mouse scroll wheel — the screen will now scroll horizontally!
This appears to be an OS X system level feature, so it should work in any app where you can scroll horizontally, though I have not tested it deeply just yet.
I don't know how well known this trick is among Mac users (it was certainly a surprise to me), but I'm already finding it indispensable in detailed graphics work.