UPDATE: Force Quit has changed in iPhone 3.0. Check out our new how-to!
Those of us who rock Mac OS X know all about the "Force Quit". For Windows users, think killing an application via Task Manager. They're both ways to shut down non-responsive or otherwise rogue applications from freezing us out or just slowing us down. For iPhone users, well, we don't have to worry about that, do we? (Remember Apple mocking Windows Mobile for multitask management?)
Well, since MobileSafari, MobileMail, and other Apple apps do multitask and run in the background, it turns out we iPhone owners do still need to worry about it. And with the App Store providing all sorts of new and potentially greedy applications to strain the more limited resources of Mobile OS X, it's certainly important functionality to have.
So what can we do? Luckily, Apple built in an solution.
Hold down the "Home" button for about 6 seconds. Your resources will then be freed up, and you'll be dropped back to the Home Screen ready for a fresh, clean start.
Faster and easier than a full reset, it can get you out of an App jam or improve the "snappiness" of your iPhone in general.
Note: if you have tabs open in MobileSafari, the cached pages will be cleared, but fret not, MobileSafari will re-load the pages off the net for you as soon as you relaunch it.
(Thanks to Antony for the screen shots)