Merriam-Webster Online provides a free, searchable on-line dictionary and thesaurus, word games, a word of the day, and many other English language and vocabulary reference tools and resources. http://www.m-w.com
The Oxford English Dictionary's Web site features a different word each day, including comprehensive etymology and usage. http://www.oed.com
World Wide Words http://www.quinion.com/wordsis Michael Quinion's language pages, including articles, the word hoard, newsworthy words, and usage notes.