Does anyone know of a resource online that lists all known English words, including plural and singular forms of a word?
There is no list of all known English words, because there are so many ways to identify what is a word and what is not a word, and so many reasons why a word may or may not be an English word. See this entertaining article in Slate summing up the problem:
WordByLetter.com seems pretty comprehensive. If anything, it seems overly comprehensive, as in I'm not sure all of those are actual English words. But if lists of words is whats you wants, lists of words is whats it has.
WordNet is much more that just a list, but you may find it useful
WordNet® is a large lexical database of English, developed under the direction of George A. Miller (Emeritus). Nouns, verbs, adjectives and adverbs are grouped into sets of cognitive synonyms (synsets), each expressing a distinct concept
Try here: an amazing service. http://wordlist.aspell.net/ Lets you create your own lists, provides some pre-made ones, too. (I'm constantly amazed at the amount of stuff available!)
Howjsay might fulfil your needs.
It has 174,302 English words, and includes plurals, conjugated verbs, etc.
It also includes rare and archaic words - I can't find any omissions.
It also tells you both British and American pronunciations.