The Linguistic Society of America: The major linguistic association, with international membership. There you can find a directory of members, information about Language, the LSA's quarterly publication, and a directory of Programs in Linguistics in the U.S., among many other things.
The International Phonetic Association: This site contains the latest version of the IPA (International Phonetic Alphabet). You can also find links to sites where you can download IPA fonts for your computer.
American Association for Applied Linguistics: An association focused on applied language issues, including language education, bilingualism, literacy, language policy and others.
Society for Pidgin and Creole Linguistics
The Linguist List: The most comprehensive source of linguistic information. Also a free service there that lets you ask a professional linguist questions about linguistics, languages, grammar, etc. Highly recommended, for students and teachers alike.
Lexicon of Linguistics: Great glossary of linguistics terms.
Glossary of Linguistics Terms: Another good glossary.
Linguistics Meta-Index: A list of linguistics resources on the web
Do You Speak American? Website for PBS series of the same name. Great information about different dialects of American English, plus fun games and quizzes
The Organization of Dialect Diversity in North America: A paper by William Labov, world-renowned authority on American English dialectology, complete with maps from the Phonological Atlas of North America.
The Phonological Atlas of North America: Links you to maps which reflect phonological change in North American dialects.
African American Vernacular English Information Page: Maintained by the Center for Applied Linguistics.
The Linguist List's Ebonics Page
Ethnologue: Languages of the World: Catalogue of the world's languages, it is a mine of information about all languages - from where they are spoken to the linguistic family they belong to. With indexes.
LoveLanguages: Links to information on about 200 languages.
The List of Language Lists: Links you to a directory of listservs, alphabetically arranged by language.
Merriam-Webster OnLine: Definitions available online.
Roget's Internet Thesaurus
Survey of English Usage. Information about the International Corpus of English.
British National Corpus: BNC. Information about and links to many other corpus pages.
The Rutger's Optimality Archive: Papers on Optimality Theory.
Linguistic Fun: A compilation of sites.
Educational Leadership Resources: A compilation of sites with information on Sociolinguistics, Historical Linguistics, Phonology, Syntax, Semantics, and Applied Linguistics. Also contains links to information about famous linguists and careers in Linguistic Anthropology.