iPhone Voice Control

One of the not-so-obvious features of the iPhone 3GS and iPhone 4 is Voice Control.  One of the reasons it's easy to forget about, if you know it's there at all, is that there is no app icon for it and there is very little in the way of settings for it.  Voice Control is extremely useful in the car, when you don't want to take your eyes off the road to place a call or play some music.

It works quite simply: Just hold down the home key for a couple of seconds and let go.  The Voice Control screen will appear and a sound will play indicating it's time for you to speak.  Just say something like "Call John Deaux mobile" and a nice lady will reply back, "Calling John Deaux mobile".  You'll soon be talking to John Deaux without having to take your eyes off the road.  After you've talked to John long enough and want to listen to music, press the button again and say something like "Play playlist eclectic favorites".  (Substitute whatever playlist you are in the mood for.)
There are a lot more commands you can use to shuffle your music, use Genius to find songs similar to the one you're playing, find out what time it is, and even find out what song or artist is playing.  Ricky Buchanan at ATMac has put together a complete list.