Mobile Deluxe is offering a "Green Edition" of Sudoku. Players can donate game points toward planting real trees, through the Trees for the Future nonprofit. Granted, the game itself costs $2.99, and players can donate three trees total, so it still comes about to about $1 per tree. Nonetheless, if you're going to play Sudoku, you might as well plant some trees while doing so; so far, players have donated 704 trees, out of a 25,000 tree goal. The game is compatible with mobile phones and smartphones, including the Blackberry and iPhone; it is available here or at the iTunes store. (Players of Mobile Deluxe's Bliss can donate points toward breast cancer research).

If you never quite understood the whole Sudoku phenomenon but would be willing to plant some trees to offset your crossword puzzle habit, paper-use habit or even carbon-production habit, you can make a donation directly to Trees for the Future ($5 minimum), where trees cost 10 cents each; $5 (tax-deductible!) plants 50 trees. The nonprofit has a four-star ("exceptional") rating from Charity Navigator.