1. One Look Reverse Dictionary. You describe an idea and this dictionary supplies the appropriate word that encompasses that concept.
2. Tip of My Tongue. Inputting partial words and meanings, this handy search engine helps you find the word you can't quite remember.
The average for adult native English speaker is between 20,000 and 35,000 words. My score was 30,900 and I was a little disappointed- I'm going to try and make the effort to look up words I don't know when I'm reading.
Another surprising fact I learnt after taking the vocab quiz, is that, on average, adults learn one new word a day, until middle-age when vocabulary growth basically stops.