My favorite version is, surprisingly, <a href="http://en.wikipedia.org/wiki/Gameloft">Gameloft's</a> <a href="http://www.gameloft.com/mobile-games/platinum-sudoku-2/">Platinum Sudoku 2</a>.  But that's cellphone only?

<h3>PC versions</h3>

<ul>
<li><a href="http://www.sudoku.gs/">Sudoku.gs</a>
<li><i><a href="http://directory.google.com/Top/Games/Video_Games/Puzzle/Browser_Based/Sudoku/">DMOZ</a></i>
</ul>