This estimates the number of words you know in English. The server is a bit slow, but help the researchers gather more data! They especially want respondents under the age of 15.
I'm at 30,000 (not amazing but not terrible--all that nerding out as a child helped, I'm sure), according to this. I'm curious about the accuracy of their algorithm, though, so I'm going to read more about it when I'm no longer pretending to work, or you guys could tell me if it's full of bullcrap. ;)