Here are a list of sites that help when it comes to looking up, learning, or researching the English language.
American Heritage Dictionary
The American Heritage Dictionary of the English Language is, as the name suggests, a dictionary for American English. The online edition is quite excellent but not terribly comprehensive.
The Cambridge equivalent of the Oxford English Dictionary is unfulfilling and sparse on any information beyond the basic meaning of a word.
A dictionary published by HarperCollins with the Collins being William Collins of Scotland. The online edition is pretty good with audio pronunciation guides and etymologies.
The Longman Dictionary of Contemporary English is another dictionary of British origin. The online edition is a little stingy with its information, but can still be a good resource sometimes.
Macmillan is another publishing firm with roots in Scotland; its online dictionary is very basic and lean on information.
The Macquarie is an Australian English dictionary and requires a subscription (boo!).
Of Noah Webster fame; primarily for the American English variant. The online version is relatively sparse in comparison to the dead tree Webster.
One of the best dictionaries in the world, the Oxford English Dictionary's official online version is not free, but well worth paying for. A free alternative can be found with Oxford Dictionaries Online, but it's not nearly as comprehensive.
Wikipedia's open dictionary. It is not always reliable. But it does not restrict access to words and provides etymology, meaning and usage information along with synonyms, antonyms and other interesting titbits including links to Wikipedia pages where applicable. It also services languages other than English.
Wordnik is a unique resource which displays entries from various dictionaries in a single location along with practical examples of usage, etymology, synonyms, and a host of other related material. But what's best of all with this site is its layout - clean and refreshing!
Online Etymology Dictionary
etymonline has collated etymological data from various sources into one large queryable database. While not as detailed as I might like, it is invariably correct.
Forvo is a community-driven database of word pronunciations which appears to include multiple languages and accents and also accommodates proper nouns such as names etc.
An uber-cool site that allows you to compare the pronunciations of words according to accents. This is maintained by the University of Edinburgh.
OneLook is essentially a word finder. It supports wildcards which while not as flexible as regular expressions, are nevertheless quite powerful. It also enables searching for words using wildcards and word meaning.
As the name suggests, this is where to go to find synonyms of words and even some phrases. All the ads can be a little annoying. But it's worth it.
Paper Rater is a free service that analyses your document and returns a report on its grammatical validity, style, vocabulary, and so on. It also comes with a plagiarism checker.
A pretty nifty plagiarism checker. It's free for basic use.
Please feel free to suggest resources that are not listed here!