Google.com/sky lets you explore the far reaches of the universe using images from NASA satellite, the Sloan Digital Sky Survey, and the Hubble Telescope.
Google nGrams is a fun tool that lets you search for words in 5.2 million books published between 1500 and 2008 so you can see how the amount they've been used changed over time.
You can add "+anything" to your email address and it will still work. This can be good for filtering emails (for example, you could set up a folder for all emails sent to "youremail+datingsite@gmail.com").
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Google recently released a new Maps game called SmartyPins for geographic trivia.