It lets you type in a word (I think it only has the most frequently used 100,000 words, though?) and will map its frequency of use in tweets (either by county or just by hotspots) across the continental US.
Here are some fun ones.
The differing distributions of “geek”, “dork”, and “nerd”:
“Canada”, “Mexico”, and “America” (I would have used “United States”, but it just takes words, not phrases):
“North” and “south” are nicely clustered around the states with those words in their names: