I thought it would be nice with a list of "undocumented features" both hw-model and sw specific.
I've bumped into a few during my searches here but maybe there are more kewl tricks out there!?
Adjust brigthness
Press scroll button in and adjust with up/down navigator button
Activate RUN command
Press and hold ACTION button. Tap and hold time, then release action button.
Activate internal use spy
The PocketPC shell has a built in tool called the 'Internal Use Spy'. Basically it gives you information about the foreground window when invoked. To activate it, simply bring up the window you want information on, hold down the action button, and tap and hold in a blank area of the navigation bar until you hear a messagebeep. Then, without lifting the stylus from the screen, release the action button and drag the stylus over the window you're interested in and mouseup. A dialog like the above will then appear, giving quite a bit of useful info.
http://ian.winprog.org/archives/000003.html
ROM update from CF card (X5)
Copy ROM image (*.img) to CF card.
Remove the backup battery and set it aside. Remove the main battery, then reattach it. Press and hold down Contacts, Reset , scroll in, and power then release both power and reset at the same time and continued holding contacts and scroll in for twenty seconds, then release.
http://tom61.arcadecontrols.com/axim_bootloader.html
Without the '.img file this will
Run built-in diagnostic utility
There's gotta be more tricks out there (or maybe even a FAQ which means I've waisted everybodys time

)