MobileRead's Download Guide for free Public Domain books. You can download this guide to your computer and transfer it to your Kindle documents folder, or surf to the MobileRead site on your Kindle web browser and download it directly into your Kindle. Over 11,000 books available here, all lovingly hand formatted by our own members.
Feedbooks Download Guide for free Public Domain books.
Manybooks.net for free Public Domain books (no download guide) and also free audiobooks.
1001Books - created by a MR member and Kindle owner (Daithi) to provide links to Public Domain books available at MobileRead or Feedbooks that are in formats usable on the Kindle - very easy to use.
Books on the Knob -- the site says "Bargain reads, book reviews, the Amazon Kindle and some games, technology and computers tossed in now and then."
Project Gutenberg The biggest creator of Public Domain ebooks on the net. You can download books wirelessly onto your Kindle from the site.
Puzux for interactive sudoku and crossword puzzles on your Kindle.
Internet Archive for free books and texts.