Resources on the English language

This is an annotated link list collected by Jukka Korpela. It aims at quality, not quantity.

WWWebster Dictionary by Merriam-Webster Online: Probably the largest general English-English dictionary on the Internet. With detailed explanations, pronunciation information, etymologies, and even illustrations.
On-Line English Grammar by Anthony Hughes: A concise presentation of the basics of English grammar.
Common Errors in English by Paul Brians: 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!
Elements of Style by William Strunk, Jr.: A classic written in 1918. Especially section III, Elementary principles of composition, is very valuable.
Grammar, Punctuation, and Capitalization - A Handbook for Technical Writers and Editors by Mary K. McCaskill: Good detailed information about the topics. (But notice that the punctuation rules of English vary to some extent from one authority to another.)
alt.usage.english resources: Contains the alt.usage.english FAQ Answers to Frequently Asked Questions list for the alt.usage.english newsgroup, and supplements to it and related documents. (The FAQ is not very systematic, and it is technically one large file, but it contains detailed answers to many questions.)
The Word Detective: A collection of answers to questions about the origins of words and phrases, with a nice alphabetic index.
Guide to Grammar and Style by Jack Lynch: 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.

The list above is mainly for my personal use, but perhaps it is of some interest to others, too. For a larger collection of links on the English language, selected by a professional, see the Language Help Page by Ruth Vilmi.