One of the most annoying things about the Apple TV is using its on-screen keyboard -- for instance, when entering WiFi information, usernames, or searching for YouTube videos. The process of moving character-by-character and row-by-row is slow and tedious. If you've got an iPhone or iPod touch and Apple's free Remote application, however, you're holding the solution to this problem in your hand: just fire up Remote any time you see a keyboard input screen on your Apple TV, and the keyboard will appear on your iPhone (or iPod touch).
While I wouldn't go so far as to say that this one feature justifies the cost of an iPhone or iPod touch, if you use your Apple TV a lot, it is an amazing timesaver to be able to use a "real" (real virtual?) keyboard instead of the Apple TV's onscreen version.
I first heard about this from an anonymous Macworld reader via email, as well as in this entry on Daring Fireball. (Macworld also covered it in this iPhone Central write-up.)
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080811053346410