Are you looking for a word that isn't in your dictionary? Or perhaps you just don't have your dictionary on you at the moment. It doesn't matter! Just go to one of these sites to find the word you're looking for.
In addition, many of these online dictionaries have information on pronunciation, synonyms (words with a similar or the same meaning) and related words, and etymology (where/when the word came from originally). Some sites even have crossword puzzles and other word games to help you with vocabulary.