Your iPhone's home button has a whole bunch of uses that vary depending on your location and how many times you press it in succession. Here's a look at (practically) all of them.
Pressing the home screen button once while on your first home screen page will take you to the search page. When on the search page, pressing it will take you back to the first page. If you're on the second, third, fourth page (or higher), a single press of the home button will return you to the first page. When your phone is locked, a single tap will bring up the lock screen. If you're in an app, pressing the home button will take you to the home screen (surprise!). Finally, if you press and hold the home button you'll initiate voice control.
Pressing the home button twice while on the lock screen will bring up your iPod controls. Doing that on any other screen will bring up the multitasking bar where you can access apps and settings.
Triple press is reserved for accessibility options. You can set them as you like by going into Settings -> General -> Accessibility -> Triple-click Home.
For jailbreakers, you can program the home button to do even more with Activator.
Shortcut of the Day highlights a great shortcut on various platforms from the desktop to mobile devices to the web. You can find more shortcuts here.