http://www.natcorp.ox.ac.uk/ The British National Corpus contains 100 million words annotated with parts of speech. The corpus allows you to search for specific words or phrases from databases of actual English usage, which is helpful for checking for usage patterns.
http://dictionary.cambridge.org/ Cambridge Dictionaries Online are corpus-informed dictionaries for English language learners at all levels.
http://public.wsu.edu/~brians/errors/errors.html This site covers commonly misspelled words and confused expressions.
http://www.collinsdictionary.com/dictionary/english-cobuild-learners This dictionary is based on a “corpus” — a collection of British and American newspapers, books, TV programs, real-life conversations, etc., providing useful information about the English language and full-sentence definitions.
http://www.merriam-webster.com/ Merriam-Webster Online contains over 225,000 entries and is based on the print version of Merriam-Webster’s Collegiate Dictionary.
http://onelook.com/reverse-dictionary.shtml OneLook’s reverse dictionary allows you to describe a concept and get back a list of words and phrases related to that concept.
http://www.oed.com/ The Oxford English Dictionary contains over 355,000 entries.