Double-tapping is actually pretty rare on the iPhone. It’s not like the Mac or Windows, where double-clicking the mouse means "open". On the iPhone, you open something with one tap.

A double tap, therefore, is reserved for three functions:

In Photos, Google Maps, and Safari (the Web browser), double-tapping zooms in on whatever you tap, magnifying it by a factor of two.

In the same programs, as well as Mail, double-tapping means, "restore to original size" after you’ve zoomed in. (Weirdly, in Google Maps, you use a different gesture to zoom out: tap once with two fingers. That gesture appears nowhere else on the iPhone.)

When you’re watching a video, double-tapping eliminates or restores letterbox bars.

See, the iPhone’s screen is bright, vibrant, and stunningly sharp. It’s not, however, the right shape for videos.

Standard TV shows are squarish, not rectangular. So when you watch TV shows, you get black letterbox columns on either side of the picture.

Movies have the opposite problem. They’re too wide for the iPhone screen. So when you watch movies, you wind up with letterbox bars above and below the picture.

Some people are fine with that. At least when letterbox bars are onscreen, you know you’re seeing the complete composition of the scene the director intended.

Other people can’t stand letterbox bars. You’re already watching on a pretty small screen; why sacrifice some of that precious area to black bars?

That’s why the iPhone gives you a choice. If you double-tap the video as it plays, you zoom in, magnifying the image so that it fills the entire screen.

Part of the image is now off the screen; now you’re not seeing the entire composition originally broadcast. You lose the top and bottom of TV scenes, or the left and right edges of movie scenes.

If this effect winds up chopping off something important - some text on the screen, for example - restoring the original letterbox view is just another double-tap away.

Save images in safari and mail

Just touch and hold an image in Safari or Mail, an action sheet will be presented to enable you to save the image. The image will get stored in the "Saved Photos" library of the Photos app.

Access iPod controls or phone favorites instantly

Go to "Settings -> General -> Home Button". Double taping the home button goes to "Home", "Phone Favorites" or "iPod Controls".

Forward/rewind music & video

Press and hold the skip forward/back arrows to fast forward or rewind rather than skip tracks. Also, press on the album art to manually FF/rewind.

Use iPhone as a Modem

You can use iPhone 3G or later as a modem to connect, or tether, your computer to the Internet. You can connect iPhone to your computer using the Dock Connector to USB Cable, or via Bluetooth.

In Settings, choose "General -> Network -> Internet Tethering".

Slide the Internet Tethering switch to On.

Connect iPhone to your computer:

USB: Connect your computer to iPhone, using the Dock Connector to USB Cable. In your computer’s Network services settings, choose iPhone. On a Mac, a pop-up window appears the first time you connect, saying "A new network interface has been detected". Click Network Preferences, configure the network settings for iPhone, then click Apply. On a PC, use the Network Control Panel to configure the iPhone connection.

Bluetooth: On iPhone, choose "Settings -> General -> Bluetooth" and turn on Bluetooth. Then refer to the documentation that came with your computer system software to pair and connect iPhone with your computer. When you’re connected, a blue band appears at the top of the screen. Tethering remains on when you connect with USB, even when you aren’t actively using the Internet connection.

Monitor your cellular data network usage: In Settings, choose "General -> Usage".

Earbud Cord Switch

Without close inspection, you’d have a hard time telling the iPhone’s white stereo earbuds apart from a regular iPod’s-but don’t get them mixed up. The iPhone’s earbuds have a tiny, embedded clicker/microphone partway down the right earbud cord.

That’s right, "Clicker/Microphone". The tiny bulge is the microphone for phone calls. But if you pinch the bulge, you’ll find that it clicks.

Pinch once to answer an incoming phone call. Pinch twice to dump the call to voicemail. (You can also tap or double-tap the "Sleep/Wake" switch on top of the iPhone for the same functions.)

During music or video playback, pinch once to pause the music; pinch again to resume playback. During music playback, double-pinch to skip to the next song.

I think I’ve found a way to get the loudest speaker volume on an iPhone or have stupidly messed up my tiny speaker on my iPhone. But the honest true, it works NICE & CLEAR…

I was playing around with trying to get my iPhone speaker volume louder and my ringtones, but I wasn’t happy with the different changes on my "SystemSoundMaximunVolume.plist" so, I grabbed a needle and sticked it into every single tiny hold on the bottom left corner of the iPhone. I think it was dirty in there or something because now this is EXACTLY how I wanted it since day one… LOUD!!! I really hope I didn’t mess up the speaker though.

Has anyone done this? Maybe my speaker was dirty and it’s now cleaned up? or I have sticked the needle into the speaker without messing it up and it made some air ways and made it louder? Whatever it is, I now love the speaker phone.