For each vocabulary answer you get right, FreeRice.com donates 10 grains of rice through the World Food Programme to help end hunger. Cool!
Here are the 100 most common SAT words. Play a matching game to see what definitions you know and which you need help with.
The site specifies that this game is played with GRE vocabulary, but really, the vocab words are useful for high schoolers on up.
Building a Better Vocabulary
Here, play different matching vocabulary games with words that are truly GRE level.
Sort through vocabulary games involving context clues, analogies, antonyms, synonyms and more. Appropriate for middle-schoolers on up.
Yahoo Word Games
Yahoo offers all different sorts of word games from Literati to Scramblers to Boggle. Everything is rated on a 5-star basis, so you'll know what other people think about the games before you try them.