There is a hidden menu item in the font menu [aA] of all Kindles that toggles between left and full text justification. This menu item can be enabled and set from the default "full" to "left" so that most properly formatted Kindle ebooks can be read with a ragged right margin.
There is an old thread in the developers section that details how to do this on the Kindle DX using the usbnetworking hack. The instructions that follow detail editing a simple text file in the user mounted section of the Kindle and restarting. No hack installation required. Much simpler.
These instructions have been tested on a Kindle 3 but should work with all versions of the Kindle.
Turn on your Kindle and press the Home button (these instructions won't work if you mount your Kindle while in a book)
Plug in your USB cable so that the Kindle mounts the user section
Edit the file system/com.amazon.ebook.booklet.reader/reader.pref in a UNIX end-of-line aware editor. The system folder is at the same folder level as the documents folder. There is a good discussion on how Windows users can edit this file in the thread "Changing the reader font without a hack" by NiLuJe. http://www.mobileread.com/forums/showthread.php?t=99355
Add the following line to the end of the reader.pref file:
Eject your Kindle
From the home screen, restart your Kindle by using the following sequence of menu items:
Home, Menu, Settings, Menu, Restart
Done! You can now press the font button [aA] and you will see a menu item called "Justification" with options of "left" or "full". This change is persistent. You can modify any other font menu item and the justification should remain set.