An enjoyable and useful vocabulary test (via Anatoly) that gives you a bunch of words, asks you to check whether you know them (“Don’t check boxes for words you know you’ve seen before, but whose meaning you aren’t exactly sure of”), and extrapolates your total vocabulary size. The About page says:
TestYourVocab.com is part of an American-Brazilian research project to measure vocabulary sizes according to age and education, and particularly to compare native learning rates with foreign language classroom learning rates. … The site provides accurate results for virtually everyone, from very small children (with answers inputted by parents) to professional linguists. It can calculate vocabulary sizes from less than 100 words to more than 40,000 words. For those interested in exactly how it works, please see the nitty-gritty details page.
I’m a sucker for these things even when they’re done in a haphazard fashion by untrained people, but this is for Science! So give it a try and (if you feel like it) pass it on; they say: “We especially need participation from children and teenagers, where the biggest vocabulary growth occurs, so families are key.”