This one might not be new, but it is pretty useful. When you are designing a Smart Playlist in iTunes, you sometimes want to specify the same criterion a number of times -- Playlist is Often and Playlist is Rare and Playlist is New, for example. If you click the Plus (+) button, you're offered an empty Artist criterion. You then have to manually select the criterion type you're after in the pop-up menu (and with iTunes not being keyboard controllable ... what a waste of time!).
But a nice feature is hiding in this dialog: Just option-click the Plus button next to the criterion you want to replicate, and iTunes will create an exact copy. Enjoy!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061214093805483