FreeRice is a not-for-profit organization, but they do place advertisements at the bottom of the web site as you play the vocabulary game. Funding from the advertisers is what pays for the rice that gets donated.
An added benefit, according to FreeRice:
After you have done FreeRice for a couple of days, you may notice an odd phenomenon. Words that you have never consciously used before will begin to pop into your head while you are speaking or writing. You will feel yourself using and knowing more words.