ESL and EFL. There are quizzes, word games, word puzzles, proverbs, slang expressions, anagrams, a random-sentence generator and other computer assisted language learning activities. Even though the primary focus is ESL, native English speakers may also find some interesting things here.
I use the Minimal Pair Practice (uses Flash and MP3 Files) at http://www.manythings.org/pp/ to raise language learners’ awareness of minimally different vowel and consonant sounds in English (ah vs. uh, r vs l etc). If students can’t hear the difference, they’ll never produce them differently.