Introductory Bibliography - A beginner's biblilography of general linguistics, with sections for reference works, standard and classic early studies, works by and about Saussure and Chomsky, and miscellaneous specialized studies.
The LINGUIST List - The most complete collection of resources on the web for all branches of linguistics. It is centered around the LINGUIST e-mail list, which can be searched online and also offers links to material such as topic-oriented sites, dissertation abstracts, and book reviews.
Ask a Linguist (from the LINGUIST List) - An e-mail answerbag for linguistic questions fielded by a panel of professional linguists. You can post a question or read archived discussions.
iLoveLanguages - Formerly The Human Languages Page, a comprehensive catalog of language-related Internet resources with over 1600 links, strongest on links to particular languages. It can be slow, but it is searchable if you know specifically what you're looking for.
The Voice of the Shuttle (Search for "linguistics," "language," or other terms) - Part of a massive website of academic links, the VoS Linguistics home page contains numerous useful and interesting links, both serious and whimsical, including "The Klingon Language Institute."
Summer Institute of Linguistics Home Page - Located at the International Linguistics Center in Dallas, this site's focus is the study of minority languages and cultures around the world. There are numersous links to resources for the study of linguistics, anthropology, translation, literacy, language learning, and computing.
Oxford English Dictionary, 2nd Edition (OED2) By subscription only. - This monument to historical linguistics, recently revised for the first time in 70 years, is still a basic tool for linguists. An older version of the web site is available here
The Merriam-Webster Online Dictionary - The collegiate dictionary, good also for comparison with the OED.
Grammar and Style Notes - Jack Lynch of the University of Pennsylvania provides this online handbook of traditional "prescriptive" grammar. For a radically different approach to grammar, see the section on Chomsky, below.