This is an annotated link list that aims at quality, not quantity.
An extensive dictionary, covering both British and American English.
A large general English-English dictionary on the Internet. With detailed explanations, pronunciation information, etymologies, and even illustrations.
Another extensive dictionary, with rich information about phraseology.
Contains a large number of etymologies for English words, with emphasis on international (e.g., Latin-based) words.
An encyclopedic dictionary of English, from the early 1900s, considered by many to be the finest ever produced in the US. Contains detailed etymology notes.
A large site with online reference books, such as The American HeritageŽ Dictionary of the English Language.
A concise presentation of the basics of English grammar.
A most valuable practical tool for understanding and avoiding common errors. Check the index of errors, and you'll surely find several you tend to make!
A classic written in 1918. Especially section III, Elementary principles of composition, is very valuable.
Good detailed information about the topics. (But notice that the punctuation rules of English vary to some extent from one authority to another.)
A collection of answers to questions about the origins of words and phrases, with a nice alphabetic index.
Well-written comments on words and phrases, especially warning against boring style. Example:
Sentences beginning "It is interesting that" or "It is significant that" are usually as far from interesting as can be. Don't simply state that something is interesting: show it.