The Chessboard/Checkerboard Display uses JavaScript to display the board state for a game of chess or checkers, and to update the board according to your input. You can use this page to play these games with a friend via e-mail, chat, or any other kind of textual correspondence. After you take your move, send your opponent the new URL of the page. They can visit the URL, see the board, take their move, and then reply to you with their updated URL. Alternatively, you can exchange only the "Board code" instead of the full URL; if you receive a board code from your opponent, copy and paste it into the Board code field, then click Display.
The page knows nothing about the rules of chess or checkers; it simply displays certain graphics in a certain arrangement. You should review each move to make sure it is legal. The advantage of this is that you are free to play variant game types. See ChessVariants.com for some ideas. (And use this link to generate a random starting position for Chess960.)
The page is released publicly under the terms of the GNU GPL. If you intend to modify it, you'll probably want to consult the symbols file to figure out what all the variables and such mean.