Google is holding a contest to give away a bunch of Nexus S's. You need to follow @googlenexus and solve the puzzles they give you. Google will release new puzzles around 9am PT and followers are only allowed one entry per day. If you submit multiple entries, your subsequent guesses will be disqualified.
This contest is only open for US, UK and Canadian residents (minus Quebec). Find the official rules in the source link. (And if you're looking to win a Nexus S without all the hassle of rules and Twitter, we're giving one away, too.) [Google]