http://englishpage.com (This site is excellent in terms of interactive games and what not)
http://aliscot.com/bigdog (great for vocabulary, sentence structure, and composition structure)
http://www.englishcorner.vacau.com (also an excellent site for interactive learning)
www.edhelper.com/vocabulary/htm (has grades 1-12 at top; at the bottom, however, you can build vocabulary puzzles/games with own set of words--great resource for teachers)
http://grammar.ccc.commnet.edu/grammar/vocabulary.htm (advice for building vocabulary and other grammar issues--from a drop-down menu)
http://englishclub.com (ton of vocab links)
Word Safari: Vocabulary Expeditions -
A word guessing game that helps you expand your vocabulary while exploring remote cyber trails to other Web sites.
*Excellent resource to enhance vocabulary in an entertaining Safari setting. Guess a word correctly and track it via Fresh Tracks linking into the World Wide Wilderness and other contexts. Expedition Gear (other sites devoted to words) includes dictionary, thesaurus, word pitfalls and notorious confusables, and a spelling test. Selected as one of Writer's Digest's 101 Best Web Sites for Writers.
Plumb Design Visual Thesaurus -
Explore words and their various threads of meaning relationships within the English language by creating a spatial map of linguistic associations.
*A must for anyone who prefers information presented spatially. Search word synonyms either from a 2-D or 3-D perspective by selecting words and setting the map in motion. Then observe the automatic linking of words to related words. Built with data-animation technology, this site is a refreshing alternative to Roget's classic thesaurus.
The Idiom Connection is a collection of English idioms and quizzes, arranged either alphabetically or topically. For instance, you can scroll through the alphabetized categories to browse through the information, or you can choose a particular area of interest, such as color idioms, food idioms, medical idioms, or number idioms.
*So you think you understand the English language! This site is sure to enlighten and to entertain you. Each idiom is defined and then used in the context of a sentence.
Another useful site is http://dictionary.cambridge.org, Cambridge Dictionaries Online, which provides a dictionary of idioms. Type in a particular idiom to go to the phrase listed alongside other idioms using the same key word (ex. bird in bush, eat like a bird, etc.), its definition, and its use in a sentence.
Merriam-Webster Online Language Center -
Quick access to an online dictionary and thesaurus. To have fun while building your vocabulary, play some of the word games and sign up for the Word of the Day.