Google Books Ngram Viewer: When you enter words or phrases into the Ngram Viewer, it displays a graph showing how often they occur in a corpus of books over the decades. Selected languages available.
Lexicon of Linguistics: Definitions of linguistic terms.
Linguistic Resources on the Internet: SIL International describes this as "a topically organized list of resources elsewhere on the Internet that may be of interest to the linguist." Covers the principal categories of linguistic research.
Omniglot: The Online Encyclopedia of Writing Systems: A guide to over 180 different alphabets, syllabaries and other writing systems.
Summer Institute of Linguistics Glossary: Definitions and illustrative examples of linguistic terms and concepts.