I used it to help my mom convert her American recipes to metric units, including trickier queries like "how much is 1/2 cup of butter in grams?" for which you need to know the density of butter.

And problem sets of course, because typing in physical constants and converting units is annoying.

Oh, and http://news.ycombinator.com/item?id=3521445 ("orbital velocity of ISS in mach").

Anytime I need a result that requires knowledge of some quantity, I usually try Wolfram|Alpha first.