Where has this game been all my life? It's like a word find, only with numbers. So addictive, it's frightening! Do not start one of these at work.
I like the site above because it has an RSS Feed, plus this site will let you create the puzzles in PDF--perfect for saving & e-mailing later.
Here's a list of some of the better sites supporting the game. And here you can download a free Palm app that generates & solves puzzles.