Igor Skochinsky at Reverse Everything has some fun Easter Eggs for the Kindle (tested them on the Kindle International Edition). All the below is Igor’s work and time and effort.

From the Home Screen:
Alt-Shift-M Minesweeper
Alt-Z rescan picture directories
Alt-T show time

Alt-Shift-R reboot Kindle
Alt-Shift-. restart GUI
Alt-Shift-G make screenshot
due to an implementation bug, screenshots can only be stored on SD card, not the main storage. A gif file is saved in the card root.
Shift-Sym start demo
Enabled only if allow_demo=true is passed on the Java commandline. Needs a special demo script present on the SD card.

Off the Settings Screen (but you knew that already!)

411 show diagnostics data
511 run loopback call test
611 diagnostic data service call

The bit he notes about the picture viewer is quite cool:

I’m not sure why Amazon didn’t make it public (maybe because paging is kinda slow), but there is a basic picture viewer in Kindle.
To activate it:
1) make a folder called “pictures” in the root of Kindle drive or SD card. Kindle also checks for “dcim” made by cameras.
2) put your pictures for a single “book” into a folder inside that. The subfolder name will be used as the “book” name. Supported formats are jpg, png, gif.
3) in Home screen press Alt-Z. A new “book” should appear. Open it to view your pictures.
4) In the local menu you can toggle dithering, resize to fit and full screen mode.

Good to know! There’s a lot more info there about the file system structure etc, info that is useful if you pursue the discussions over at OSCON’s Ignite Allegedly, there’s also a GPS-linked item available in the browser…

Jesse Vincent gives a blow-by-blow run-down of the inner workings of the Kindle over at his blog.