FreeRice

The name really says it all: this game is a quiz program presenting four multiple-choices to a question (originally synonyms to an English word, which can be read out loud by the site) with the player clicking on, ideally, the correct answer. With every correct answer, not only does their game level increase (resulting in more difficult questions being offered, up to level 60... though most players are unlikely to surpass level 50) but twenty grains of rice (depicted visually as they fill a wooden bowl) are secured through the site's sponsors for eventual distribution to impoverished citizens of poor countries through the UN World Food Program. Incorrect responses will lower a player's level after displaying the correct answer, restoring players to a level where they can answer questions correctly (the offending question coming back up in the queue to give players an opportunity to demonstrate what they've learned!) and maintain the accumulation of free rice for hungry folks.

Though the site began solely offering tests for English Vocabulary (inspired by the developer's overlooking his son's SAT cramming) and later Grammar, it has since expanded to include categories for other languages (French, German, Italian, Spanish), Famous Paintings, Chemical Symbols, World Capitals and Multiplication Tables. There may be more effective and more fun ways of studying, but few as well-intentioned!