freerice.com is a click-to-donate website where players challenge themselves with tests of knowledge (including, most importantly for this forum's crowd, English vocabulary and grammar) while supporting the World Food Programme.
The adaptive learning system keeps you at whichever level gives you the most efficient learning experience
3 correct answers in a row bring you up one level in difficulty out of 60
1 incorrect answer brings you down one level, shows you the correct answer to the question that you got wrong so that you can learn the correct answer for later, and gives you a repeat of that question shortly after so that you can cement your memory of the correct answer that you were given
and the advertisements on the sidebars pay for the website to donate 10 grains of rice for every answer you get correct.