FreeRice! I liked the concept of this site very much.. Lakshman was the one who introduced me to this. Its basically a vocabulary game, A word is given and we have to select the definition from a list of specified options. There are different levels, the level gets tough as we progress. The concept I was talking about is this "For each correct answer we give, the sponsors of FreeRice website (the ads we see on the bottom of the site) donate 20 grains of Rice to the UN Food program".
At the same time this site is good for those who want to improve their vocabulary which will prove quite helpful in various competitive exams such as GRE, CAT etc. The UI is catchy at the same time. You can see a bowl where some 20 grains get added after every question. I liked it.. Hope you enjoy it too.
For more details, please refer to the FAQ section of the site.