Natural Languages

Terralingua. Site devoted to linguistic and biological diversity.
Summer Institute of Linguistics. Specialises in minority languages. Also has downloadable IPA fonts.
Native American Language Resources on the Internet.
The Association for Linguistic Typology.
Essentialist Explanations. Witty (verging on abusive) collection of descriptions of languages e.g. "English is essentially bad Dutch with outrageously pronounced French and Latin vocabulary". Compiled by John Cowan.

Constructed Languages

The Klingon Language Institute.
The Elvish Linguistic Fellowship. No kidding.
Lojban. Constructed language based on predicate logic.

General Linguistics

The Linguist List. Prestigious e-zine and resources collection.
Systemic Functional Linguistics
Linguistics Materials on the Web. Comprehensive links list maintained by the Linguistics department at Rochester.

Cognitive Linguistics

Sociolinguistics

Explore Sociolinguistics. Nice introductory site from the University of Oregon.
The Ladies' Room Very nicely presented site devoted to gender differences in communication.
Berkeley Women and Language Group

Applied Linguistics / TEFL

Resources in Applied Linguistics. Useful site maintained by Surrey University's English Language Institute.
The Language Teacher. Web journal published by the Japan Association of Language Teachers.
The Internet TESL Journal
Many TESL/TEFL/TESOL/ESL/EFL/ESOL Links

Linguists' Home Pages

Noam Chomsky. Yes, the man himself. 
Deborah Tannen
Mark Turner.  Cognitive linguistics.  
Nick Nicholas. Incredibly prolific linguist whose less academic achievements include translating Shakespeare into Lojban and Klingon.
Nebiye Petek Kurtboke. Turkish-Australian corpus linguist.