Google books has scanned every book they can get their hands on which is a bit over 5.2 million of them. With that kind of data in a searchable format you can do some interesting things. The Google Books Ngram Viewer is the result. This is what it does, but you'll figure it out faster if you just mess with it yourself. I've played with it, and here are some interesting finds that have value.

You can track when a new word originated and its popularity over time:

• bootlegger
• gatling
• movie
• serial killer
• velcro

You can see when your friend's newer and unusually spelled name got popular:

• Aiden
• Alyssa
• Jaden
• Makayla

You can see which names dominated a time period:

• James vs. John vs. Michael vs. Robert vs. William - Which male first name was used more in the history of writing?
• Barbara vs. Elizabeth vs. Linda vs. Mary vs. Patricia - Which female first name was used more in the history of writing?
• Johnson vs. Jones vs. Smith vs. Williams - Which surname was used more in the history of writing?
• Garcia vs. Hernandez vs. Lopez vs. Martinez vs. Rodriguez - Which Hispanic surname was used more in the history of writing?
• Chen vs. Kim vs. Lee vs. Wong vs. Yang - Which southeast Asian surname was used more in the history of writing?

You can answer cultural and historical questions through the changing use of language:

• Devil - Explain the Ngram using what you know of Puritan America, the Salem Witch Trials, and the Age of Reason.
• pirate - What changes in culture, technology, and language use would explain this Ngram?
• plague - What explains the largest spike on the Ngram? What must have happened on the other spikes? Does language frequency imply severity?
• obesity - What changes in culture and language would explain this Ngram?
• tyranny - What happened in history that might explain this Ngram spike in use of the word tyranny?
• vegetarian - What changes occurred in the science of nutrition and changes in ethics that might explain this Ngram?
• aliens vs. predators - Movie jokes aside, what cultural shift in langauge would cause this Ngram?
• car vs. horse - What year marked the decline of the use of horses for public transportation?
• gun vs. knife - What times in history would people use the word "gun" more?
• man vs. woman and male vs. female - Did the women's suffrage movement and the feminist equality movement make a difference on language?
• African American vs. Negro - Explain the spikes in a historical context for the use of the word "Negro" and explain the rise of a socially acceptable replacement.

You can settle age old popular questions:

• red vs. blue - Which color is more popular?
• good vs. evil - Which is more popular: good or evil?
• love vs. money - Do we write more about love or money?
• love vs. hate - Do we write about what we love or hate more?
• beef vs. chicken vs. pork - Which meat do we write about the most?
• Star Trek vs. Star Wars - Which fictional universe do we write about more?
• Atari vs. Dreamcast vs. Nintendo vs. Playstation vs. Sega vs. Xbox - Which had the greatest penetration into our language for its time?

We can search for when a historical figure was being talked about and when they were relevant:

• Cesar Chavez
• Abraham Lincoln
• Martin Luther King
• Malcolm X
• Robert Oppenheimer

Really interesting comparisons:

• 1st vs. 2nd - What world event, invention, or cultural change would explain this Ngram? (Hint)