While not the most impressive or fun app, AT&T myWireless Mobile is essential for U.S. iPhone users because it is AT&T’s official app for checking your account status and changing some settings. You can pay your bill, check usage, and even add some features to your account.
Related: How do you sign up for Find My iPhone?, MacMost Now 275: 11 Free iPhone Apps To Get You Started, Access Mobile Me apps from an iPad, Deleting Application Support Files, Mobile Me – Mail folders.

a sound alert when your battery power reaches less than say 20%