Wiktionary
Unknown
A collaborative multilingual dictionary whose entries carry etymologies, cognate sets and reconstructed Proto-Indo-European and Proto-Germanic roots, letting you trace a modern word back through sound changes and compare its descendants across related languages.
More resources on Historical Linguistics
The Horse, the Wheel, and Language
An archaeologist's case for locating the Proto-Indo-European homeland on the Pontic-Caspian steppe, combining linguistic reconstruction with excavation evidence on horse domestication and wheeled vehicles to explain how the language family spread across Eurasia.
etymonline.com
Online Etymology Dictionary is a comprehensive, searchable resource that traces the origins and historical development of English words, detailing word roots across languages (Old English, Latin, Greek, etc.) with concise etymologies.
The History of English
A chronological history of the English language examined through the lens of historical events that shaped the development and spread of the language from the Eurasian steppe to the entire world.
ethnologue.com
Ethnologue is a comprehensive, searchable database of the world’s living languages, featuring detailed language profiles, language-family classifications, speaker numbers, endangerment status, dialect information, and maps of geographic distribution.
Language Change
A textbook treating language change as the cumulative result of ordinary usage, working through sound change, analogy, grammaticalization and semantic shift so you can explain why particular changes recur across unrelated languages.
glottolog.org
Glottolog is a comprehensive, language-centered catalog of the world’s languages and dialects with unique Glottocodes and a historical–comparative classification. It provides extensive bibliographic references and links to descriptive sources (grammars, dictionaries, wordlists) for each language.