JesseRosenthal’s sudoku.el is available from his page: “A major mode for playing sudoku puzzles on emacs. This allows you to download new puzzles from on the fly. In addition, roughly 200 puzzles are included. You can set four different levels: easy, medium, hard, and evil.”

KimStorm posted a solver on gnu.emacs.sources:

The format of the web source has changed, resulting in an error when you try to load a new puzzle from the web. To fix this, change line 1300 to this:

    "<FORM NAME=\"board\" METHOD=POST ACTION=\"./\" STYLE=\"margin:0;\">"))
    (11/29/10 --- kc5gmr)

Also check out this sudoku on steroids with bunch of features: