Found this on my hard drive. There's a distinct possibility that someone posted it ages ago but I can't remember

FIRST

The Emergency Number worldwide for Mobile is 112. If you find yourself out of the coverage area of your mobile; network and there is an emergency, dial 112 and the mobile will search any existing network to establish the emergency number for you, and interestingly this number 112 can be dialed even if the keypad is locked. Try it out.

SECOND - Have you locked your keys in the car?

Does your car have remote keyless entry? This may come in handy someday. Good reason to own a mobile phone: If you lock your keys in the car and the spare keys are at home, call someone at home on their mobile phone from your cell phone. Hold your mobile phone about a foot from your car door and have the person at your home press the unlock button, holding it near the mobile phone on their end. Your car will unlock... Saves someone from having to drive your keys to you. Distance is no object. You could be hundreds of miles away, and if you can reach someone who has the other "remote" for your car, you can unlock the doors (or the trunk).
Editor's Note: It works fine! We tried it out and it unlocked our car over a mobile phone!"

THIRD - Hidden Battery Power

Imagine your mobile battery is very low. To activate, press the keys *3370# Your mobile will restart with this reserve and the instrument will show a 50% increase in battery. This reserve will get charged when you charge your mobile next time.

FOURTH - How to disable a STOLEN mobile phone?

To check your Mobile phone's serial number, key in the following digits on your phone: * # 0 6 #
... A 15 digit code will appear on the screen. This number is unique to your handset. Write it down and keep it somewhere safe. When your phone gets stolen, you can phone your service provider and give them this code. They will then be able to block your handset so even if the thief changes the SIM card, your phone will be totally useless. You probably won't get your phone back, but at least you know that whoever stole it can't use/sell it either. If everybody does this, there would be no point in people stealing mobile phones.