Mr. Nussbaum's site has tons of educational games on it; my favorite is called Semicolon Wars. (You like that well placed semicolon, don't you?) The semicolons are tired of being pushed aside for the more plebeian punctuation like periods and commas. You must correctly place all of the punctuation in the game to defend semicolon honor!
Other great activities on the site include the ever-popular Tony's Fraction Pizza Shop, Cash Out, and General Tso's Spicy Synonyms.