This page lists some useful English-language dictionaries.
Dictionaries list words of a language or domain. Monolingual dictionaries explain words in the same language, bilingual and multilingual dictionaries translate words into other languages.
In printed dictionaries, words are arranged in alphabetical order. For online dictionaries, word order is not an issue since words are accessed through a search engine. Of course, this assumes that one knows the exact spelling of a word. Fortunately, most online dictionaries account for spelling mistakes by suggesting similar words if they do not recognize a word.
Our favorite online dictionary is the WWWebster Dictionary, http://www.m-w.com/.
The A Web of On-line Dictionaries website, http://www.facstaff.bucknell.edu/rbeard/diction.html, lists more than 1,000 online dictionaries in 200 languages.
OneLook, http://www.onelook.com/, is a meta-dictionary, i.e., a dictionary that searches other online dictionaries.
The Collins Cobuild Student's Dictionary
Merriam-Webster's Collegiate Dictionary, 10th Edition, 1993