I've already touched on keyboard shortcuts that can be used in a BlackBerry to make navigation much more easy but there are also another set of commands which are hidden for different reasons. Standard users won't need these unless under exceptional circumstances. More advanced users may find them helpful.
A good place to start is how to clear corrupted databases on the handset. If you suspect your Calendar is corrupt and you use OTA synchronisation, you don't need to use BlackBerry Desktop Manager to clear the database. Simply open the Calendar, press the BlackBerry menu key and choose Options. Now type RSET. You will get a prompt to clear and reload the Calendar database. This also works for the Contacts database.
Whilst we are in the Contacts, you can hold Alt and type VALD to validate the data structures or RBLD to rebuild them. Open up a contact, hold Alt and type VIEW will give you additional information such as the RefID and ServerID which can be useful for troubleshooting. This also works for email messages and Calendar entries.
Want to hard reboot the device without taking the battery out or downloading a third-party application? On the Home screen, press and hold the left-hand Alt key, the right-hand Shift key (aA) and tap the Delete key. You can also change the signal bar on the home screen to a numerical value by holding Alt and typing NMLL (values between -80 and -100 are ideal). Alt + LGLG gives you the Java Event log.
Ok, that last one was seriously geeky and probably not very useful for anything but die-hard enthusiasts. Let's go for something more user-friendly. Whilst you are using the BlackBerry Browser, Alt + RVBS will display the web page source code.
Two more familiar ones are related to network locks on the devices. Go into Options > Advanced > SIM Card (or Options > Device > Advanced System Settings > SIM Card on v.6 handhelds). Type MEPD to display the handheld locks. If you have a handset unlock code, type MEP2 (you need the Alt key to type the number 2) to enter it and disable the network locks. MEP1 to MEP5 all work but only MEP2 tends to be used.
Let's end with something really useful. How much Voice and Data have you actually used on your BlackBerry? Options > Status (or Options > Device > Device and Status Information) and type BUYR. See? Easy! You can also type TEST from the Status screen to start a series of comprehensive handheld tests.
Are these helpful? Let me know and I'll post more.