Everyone should go have a look at FreeRice.com. It is a relatively new site that claims to donate rice to the World Food Program, who then distribute it to people who need it. The way they fund this, is by providing a simple (but entertaining) word game, with a small advert on each page (I didn’t actually notice the advert until it was pointed out to me). The game is a series of multiple choice questions, where you have to choose the most similar word to a given word from a list of 4. For each correct answer, they donate 20 grams of rice.
To make the game more interesting, each word has a “difficulty” associated with it between 1 and 50. It starts by asking a few random words to get an idea of what level you are at. Once it has determined a suitable level, it ask you questions from that level. If you get 3 questions right, you go up a level, and if you get a question wrong, you go down a level. This means that it adapts to your vocabulary to give you words you have a chance at getting, but not making it so easy that it is boring.