Shhh... Don't tell anyone, but while these are games, you'll actually learn something as well.
    Create online RUBE GOLDBERG Machines 
    Another RUBE GOLDBERG Inspired game 
    Free Rice--great to build your vocabulary, but it also will donate rice to people in need

    Geography Game:  This site will start with a tutorial then go to a game

    Logic Puzzles:  Lots of logic puzzles and brain games

    Learn Chinese:  A BBC site with games and audio to help you learn Mandarin Chinese

    This site is a growing compilation.  What are your favorite games to stretch your brain?