Chronology: History of English - "Chronology of Events in the History of English" summarizing the emergence and evolution of the language spanning 5 millenia.
The Computational Historical Linguistics Project - A joint research project of the Computer and Information Science Department and the Linguistics Department at the University of Pennsylvania.
Corpora and Historical Linguistics - "Historical linguistics can be seen as a species of corpus linguistics, since the texts of a historical period or a "dead" language form a closed corpus of data which can only be extended by the (re-)discovery of previously unknown manuscripts or books."
Encyclopedia.com - Grimm's law - The principle of relationships in Indo-European languages, first formulated by Jakob Grimm in 1822.
The Great Vowel Shift Web Site - An interactive introduction to the Great Vowel Shift, a major sound change in the English language. Includes sound and animation.
History of the English Language - A considerable collection of online materials related to the history of English: texts, publications, fonts and course syllabuses. Includes Pre-History, Old English, Middle English, early Modern English and the spread of English beyond the British Isles.
Indin - Independent investigations into the history of the Indo-European languages and peoples (by Valentyn Stetsyuk). Includes Turkic and Finno-Ugric material.
The Proto-Sumerian Language Invention Process - Eleven-page paper describing how the early Sumerians invented their language, with a complete lexicon of the first words in their language.
Sergei Starostin's Etymological Databases - Currently comprise North Caucasian, Sino-Tibetan, Yenisseian, Altaic, Chukchee-Kamchatkan, Dravidian, and (partly) Semitic (proto-) languages. With reconstructed protoforms (including intermediary) and the attested forms in daughter tongues for each etymon. Downloadable, searchable. Need special fonts, also available onsite.
Seshat the Scribe's Extinct Languages Library - Description of the writing systems of several extinct languages, including Egyptian hieroglyphics, cuneiform, Linear B, runes and other undeciphered scripts.