Numerous features are silently sitting inside your Amazon Kindle.

Bonus content

The main GUI and most of the back-end code is written in Java. The framework is quite elaborate and can be extended with extra "booklets".
After spending some time investigating it with JAD, I found some undocumented shortcuts, features and easter eggs. Here's a more or less complete list.

Picture viewer

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.

Keyboard shortcuts

Various undocumented/underdocumented keyboard shortcuts. I italicized most interesting ones.

Global keys

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.

Home

Alt-Shift-M Minesweeper
Alt-Z rescan picture directories
Alt-T show time

Reader

Alt-B toggle bookmark
Alt-T spell out time
Alt-0 enable/disable slideshow
Alt-1 start slideshow (if enabled)
Alt-2 stop slidehow
Alt-PageForward/PageBackward go to next/prev annotation or one "chunk" (1/20th of a book) forward or backward

Settings

411 show diagnostics data
511 run loopback call test
611 diagnostic data service call
c/e/s
126 Lab126 team members

Font List

J show/hide justification options

Picture viewer

Alt-Shift-0 set current picture as screensaver
F toggle fullscreen mode

Minesweeper

I,J,K,L up,left,down,right
M mark mine
R restart
Space open cell
Scroll move cursor up/down
Alt-Scroll move cursor left/right
H return to Home screen

Text input

Alt-Backspace clear all
Alt-H/Alt-J move cursor
(the following don't work in search field for some reason)
Alt-6 ?
Alt-7 ,
Alt-8 :
Alt-9 "
Alt-0 '

Browser

It seems there is a location capability (GPS?) in the CDMA module. I cannot check it as I'm not in USA but the following shortcuts are programmed inside the browser.
Alt-1 show current location in google maps
Alt-2 find gas station nearby
Alt-3 find restaurants nearby
Alt-4
Alt-5 find custom keyword nearby
Alt-D dump debug info to the log and toggle highlight default item
Alt-Z toggle zone drawing and show log

Audio Player

Alt-F next
Alt-P play/stop

Search commands

These command work in the search field. You can enter only beginning of the command if that's enough for it to be unique.
Public commands (always available)

@help
@web
@wiki/@wikipedia
@store
@time

Semiprivate (available but not mentioned in @help)

;dumpMessages dump current debug log into the "documents" directory
;debugOn set log level=2 and enable private commands
;debugOff set log level=1 and disable private commands

Private commands

Note: following commands are clearly not intended for end users. Some of them may damage your Kindle and void your warranty. Enter at your own risk.

`help list private commands
`7777 set version to TOPmk-xyz-77770 (to disable OTA updates?)
`voltLog <1|0> enable/disable voltage table debug
`batteryLoggingDelay set battery logging delay (in seconds)
`pppStop close WAN PPP connection
`disableIndexing
`logOpenFiles
`startIndexing
`dumpBattery
`indexStatus
`compliance
`einkAdjustments
`allocate [MB]
`log611
`reloadContentRoster
`indexForever
`downloadIndex
`consumeMemory
`terminal
`checkForUpdate
`applyUpdate
`stopIndexing
`processNowNow
`processTodo
`countUnmergedDownloadedIndexes
`dumpIndexStats
`memInfo