HEL in General
Studying the History of English [a very instructive website by Raymond Hickey (Essen University)]
The Evolution of Present-Day English [maintained by Daniel W. Mosser (Virginia Tech); still incomplete but contains a wealth of information]
University of Kansas Libraries, 'History of the English Language' exhibition ['This is an online version of a physical exhibit that was on display in Kenneth Spencer Research Library, May through August of 2017.']
The Evolution of English [a website by George Boeree (Shippensburg University)]
The History of English Language [by Sun Dexian (University of Geneva)]
The Routes of English [a website on BBC Radio 4]
British History: Ages of English Timeline [another BBC website targeting beginners]
British Library, English Timeline
University of Düsseldorf, Timelines to Visualize History ['visualizes all contents taught in the Introduction to Medieval English Studies Part I and II' at the Department of Medieval English Literature and Historical Linguistics; see the press release here]
University of Manchester, Timeline of the English Language
The History of English - How English went from an obscure Germanic dialect to a global language [maintained by Luke Mastin]
A Brief History of English [by L. Kip Wheeler (Carson-Newman College)]
Red-Letter Dates in the History of English [by Russell A. Potter (Rice University)]
Sources for the study of Scandinavian influence on Old and Middle English [by Anthony Kroch (University of Pennsylvania)]
MOOCs
Open University, Exploring the English language
Open University, English in the world today
University of Glasgow – Word Webs: Exploring English Vocabulary
Seth Lerer, History of the English language, second edition
Prof Merin Simi Raij, IIT Madras, History of English Language and Literature [also available here and on YouTube]
More General Reading
History of the English language ― Wikipedia
English History and Its Language Development [at wordinfo]
English Language History [at englishlanguageguide.com]
A (Very) Brief History of the English Language
Tracing English from its Beginning [by C. Monette (chemical engineering student)]
History of the English Language [anonymous site; English roots, Old English, Middle English, Early Modern English, Present Day English]
Changes in the English Language [another anonymous site; eras of English, English subsystems, impact of foreign languages]
History of the English Language [at englishclub.com]
A Short History of the English Language [at lukemastin.com]
A Short History of the English Language [at cryptograph.com]
A Brief History of the English Language [at studyenglishtoday.net]
A Brief History of the English Language [at anglik.net]
A Brief History of the English Language [by Patrick Lockerby, in 6 parts]
French/English Translation: An Unusual History of the English Language [a short essay at translatorsbase.com]
Key Events in the History of the English Language [a short essay by Richard Nordquist at About.com]
English Language Timeline [at angelfire.com]
The Origin and the History of the English Language [at KryssTal]
Five Events in the History of English [an essay by Philip Durkin, principal etymologist of the OED]
Simon Horobin, 'A history of English ... in five words' [an article in the Conversation]
What are the origins of the English language? ['Frequently Asked Word Questions' at Merriam-Webster Online]
Others
VARIENG: Research Unit for Variation, Contacts and Change in English [a (very productive!) research unit at the University of Helsinki]
The Diary Search Website ['a guide to the location of historical and literary sources, in the forms of Diaries and Journals, from all periods and parts of the world, which have been printed in English...']
English Bible History [by John L. Jeffcoat III]