source : http://forum.lowyat.net/topic/804188

How to check your E66 firmware version
In the home screen, key in *#0000#

How to check your E66 IMEI number
In the home screen, key in *#06#

How to soft reset your phone
In the home screen, key in *#7370#
Default password should be 12345, and the phone shall restart. Please remember to backup all your important datas, contacts and files before doing this. This will reset all settings to default, but supposedly no applications will be deleted.

How to hard reset your phone
In the home screen, key in *#7870#
Hard-reset is like formatting a drive! It does format the memory. Everything that has been installed after the first use of the phone is deleted! It will recover the memory of the phone to the state you purchased it!

How to check your Bluetooth device address
In the home screen, key in *#2820#

How to check your WLAN Mac address
In the home screen, key in *#62209526#

How to check the life timer of your phone
In the home screen, key in *#92702689#. Display the life timer information of your phone, i.e. the total time of phone call (in minutes). The only way to reset this timer is by flashing the phone. This is useful while purchasing new phones to see if the phone has been used before.

How to close programs which hung up or unable to terminate by itself
In the home screen, press and hold Home button. A list of running apps will appear on top left side of screen. To end application, just press Back button.

How to mark more than 1 item conveniently
Browse to the files which you want to mark, and press # key to mark the files. Note that this will not work in File Manager.

How to Silent your phone
To set calendar and clock alarms to snooze or to silence incoming calls, just turn the device face down.
You can also silence the phone by tapping it twice quickly. To turn on silence by tapping, select Menu > Tools > Settings > General > Personalisation > Tones > Tap to silence > On.

How to unlock your phone when slider is closed
To unlock keypad without opening the slide, press the left and then the right selection key.

How to check GPS satellite status on your phone
To check how many satellites your device has found, and whether your device is receiving satellite signals, select Menu > GPS > GPS data > Position > Options > Satellite status.

How to set Remote Lock option (useful if your phone is stolen)
If your phone is stolen or gets lost, you can lock it (and the memory card) remotely by sending a lock code via SMS. To enable remote locking and to define the text for the message, select Menu > Tools > Settings > General > Security > Phone and SIM card > Allow remote lock > Yes. Enter the remote lock message and confirm the message. The message must be at least 5 characters long.

How to Password Protect your files using built-in Zip Manager
Most currect S60 phones ship with a built in zip manager that allows us to open .zip attachments received from emails etc. In can however also be used to create a password protected archieve. Please visit the link below for the method description:
Password protect your file with Zip Manager

How to Add Operator Logo
Use a filemanager like FExplorer or SeleQ to add the folders: "c:/system/Apps/phone/oplogo". Add a .bmp picture to folder "oplogo" and restart your phone! The .bmp picture size needs to be: 97 x 25 pixels

How to Backup and Restore Messages
1) Connect your handset to PC in the “Data transfer” (Mass storage) mode
2) Go to the Private folder on your memory card
3) Open the folders in sequential manner
4) Look for 1000484b (PARENT) folder or like that under which you will find Mail (Child) or mail2 folder, you are looking for.
5) Copy that (PARENT) folder to your PC
6) After you format you memory card using handset, copy back the same folder PARENT from pc again under Private folder on memory card using mass storage mode.
7) Voilla!!! You have got all your messages as it is.