BlackBerry Secret Codes

Decibel meter.

Displays your signal strength in decibels, instead of bars. Hold "Alt" and press "N", "M", "L", "L" (repeat to reverse the effect).

Address Book file verification.

Checks the data in your Address Book for inconsistencies. In the Address Book, hold "Alt" and press "V", "A", "L", "D". Address Book data structure rebuild.  Forces a data structure re-build in the Address Book...  In the Address Book, hold "Alt" and  press "R", "B", "L", "D".

View source code. 

Displays the source code of a Web page.  In the Browser, hold "Alt" and press "R", "B", "V", "S".

Help Me! menu.

Provides crucial technical information about your device, for when seeking technical support or help in forums...
Hold "Alt" and (left) "shift", then press "H".

IMEI display.

Displays your device's international mobile equipment identity (IMEI - your serial number) on-screen... Also works from all other GSM handsets.
Type "*", "#", "0", "6", "#" on the Home screen.

"Soft" reset.

Performs a "soft" reset of your device; the equivalent of doing a "battery pull" (ie. Removing the battery for a few seconds).
Press-and-hold "Alt", then press-and-hold (left) "shift", then press-and-hold "Del".

Event Log.

Displays event logs of all the system-level events that occur on your device... Hold "Alt" and press "L", "G", "L", "G".

All of these secrets have been tested on the BlackBerry 8800 and BlackBerry Curve 8300; .

In some cases, you will need to be on the Home screen when you enter these codes...
Key shortcuts:
A or C = phonebook
S = search
F = phone profiles
W or B = browser
H = help
K = locks the keys
L = calendar
V = messages
M = messages folder
R = alarm
T = tasks
U = calculator
I = applications
O = options
P = phone

Some new additions thanks to the contributions in this thread:
T - Top of page (in browser)
B - Bottom of page (in browser)
Space - Page down (in browser)
ALT + Right Shift + Del = hard reset
D - Memo pad
U - Switch between hide/unhide in title bar (in browser)
ALT + NMLL = numbers instead of bars for signal strength.

How To Enter the Unlock Codes on a BlackBerry

All models of BlackBerry

1. Go to the menu, then Settings Menu -> Select Advanced options => and then Sim card.
2. While holding down the SHIFT key, type MEPD or (MEPPD combines for Vodafone)
3. You should now see the five categories listed lock.
4. While holding down the SHIFT key, type MEP2 or (MEPP2 handsets for Vodafone)
5. Release the Shift key.
6. You are now invited to enter the Network MEP code (it will also be the number of attempts left).
7. Enter the code
8. Enter Return / Enter
9. The device must be unlocked

T-mobile:BlackBerry Models

1. In Home screen, click the Tools icon. The Options screen appears.
2. Select Settings icon.
3. Scroll to SIM Card and select it. You should see the phone number and and identification number of the SIM card.
4. Type ‘mepd’ to display the first five levels of SIM locking.
   T-Mobile uses the network to lock network should display Active. All other states must be disabled note: For the Blackberry 7100t you press "P" twice when entering the mepd 'and' mep2 "for steps 4 and 5..
5. Type ‘mep2' to display the dialog box asking for the unlock code (Press the Alt + 2 key to get the number 2)
6. Enter the unlock code network
7. Press ENTER and reboot handheld.