Reference works

Robert Beard's http://www.yourdictionary.com
ULB Darmstadt Reference Works (restricted access for TUD members)

Academic writing

Purdue Online Writing Lab
Purdue OWL on using the APA (American Psychological Association) Formatting and Style Guide

Texts in and on linguistics

A short text on 'The History of Linguistics' by Frederick J. Newmeyer, University of Washington.

Classic readings in linguistics:
Chomsky, Noam, 1957, Syntactic Structures, The Hague: Mouton.
Fillmore, Charles. J. 1968. "A case for case." In: Bach, E., R.T. Harms. eds. Universals in Linguistic Theory. New York: Holt, Rinehart and Winston. 1–88.
Humboldt, Wilhelm von, 1988 [1836], On language: the diversity of human language-structure and its influence on the mental development of mankind, translated by Peter Heath, with an introduction by Hans Aarsleff, Cambridge: CUP.
Robins, R. H., 1997. A Short History of Linguistics, 4th ed., London: Longman.
Saussure, Ferdinand de, 1983 [1913], Course in general linguistics, edited by Charles Bally and Albert Sechehaye with the collaboration of Albert Riedlinger; translated and annotated by Roy Harris, London: Duckworth.

Linguistics online

Linse Linguistik Server Essen 

Corpus linguistics

Gateway to Corpus Linguistics on the Internet 

Corpus linguistics tutorials

Concordances and corpora tutorial at Georgetown
BabelOn an der Universität Bielefeld: eine deutsche online Einführung in Grundlagen der Linguistik und in die Korpuslinguistik

Corpora

English corpora
The British National Corpus
Do a simple search of the BNC-World online
The Survey of English Usage

German corpora
The Cosmas corpus collection at IDS Mannheim
LIMAS - Linguistik und Maschinelle Sprachbearbeitung 

Corpus queries online

The Linguist's Search Engine by Philip Resnik (a very neat interface for using the web as a corpus; includes a parser). There's a guest login if you want to check it out.

Papers in corpus and computational linguistics:

CiteSeer Scientific Digital Library

Corpus encoding

XCES
The Text Encoding Initiative (TEI) website

Linguistics glossaries

Glossary of linguistic terms by Eugene Loos et al. at SIL

Grammar references online

canoo.net on German grammar

Knowlegde representation

John Sowa

Further links

wordcount.org
Corpus tools
Natural Language Processing