HEL in General
A very instructive website by Raymond Hickey (Essen University, Germany), who also maintains Studying Varieties of English
"[I]ncludes a list of links to on-line, worldwide resources for the study of the English language and its history". Maintained by Carol Percy (University of Toronto, Canada). The older version is available here
Maintained by Daniel W. Mosser (Virginia Tech, USA). Still incomplete but contains a wealth of information.
A tree diagram of HEL, designed by Piero Morpurgo.
A website by George Boeree (Shippensburg University, USA).

By Sun Dexian (University of Geneva, Switzerland).
By Douglas F. Hasty (Florida International University, USA).
A website on BBC Radio 4.
Another BBC website targetting beginners.
By Dan Mosser (Department of English, Virginia Tech, USA). A subscription to the HEL-L discussion list is available from here.
A companion website for Jeremy J. Smith's Essentials of Early English (2nd ed., 2005). Contains OE, ME and EModE texts and their PDE translations, all published in Essentials, plus reading of the texts by the author himself. 
By L. Kip Wheeler (Carson-Newman College, USA).
By Suzanne Kemmer (Rice University, USA).
An extended version of the above website by Suzanne Kemmer.
A page at the British Library website.
By Russell A. Potter (Rice University, USA).
A very detailed resource for phonetic changes from OE to PDE. Maintained by William E. Rogers (Furman University, USA).
By Anthony Kroch (University of Pennsylvania, USA).
 HEL in Classes  
A website at the Department of English, Heinrich-Heine-Universität in Düsseldorf, Germany. Also available is an online version of its HEL class "HELO (History of the English Language Online)".
HEL course materials by Daniel Donoghue (Harvard University, USA).
HEL course materials by David Sprunger (Concordia College, USA).

A companion website of Elly van Gelderen's A History of the English Language (revised ed., John Benjamins, 2014). The website for the first edition is available here.
A companion website of the same title of the textbook by Laurel Brinton and Leslie Arnovick (Oxford University Press, 2006).
A companion website of the same title of the textbook by Dennis Freeborn (Palgrave Macmillan, 2006).
HEL course materials by Edwin Duncan (Towson University, USA).
HEL course materials by G. Steinberg (The College of New Jersey, USA).
HEL course materials by Dennis Baron (University of Illinois at Urbana-Champaign, USA).

Offers a series of short e-lectures on HEL (by Jürgen Handke, Marburg University, Germany).
 More General Reading 
 English Language History [at englishlanguageguide.com]
The same contents are also available here.
An article at Random History.

 History of English [at Mahalo] 
 History of the English Language [at englishclub.com]
 History of the English Language [by Rita Putatunda at Buzzle.com]
 A Brief History of the English Language [at studyenglishtoday.net]
 A Brief History of the English Language [by Patrick Lockerby, in 6 parts]
More or less similar descriptions of HEL...
A short essay by Earl Hunsinger at buzzle.com.
A short essay by Richard Nordquist at About.com.
An essay by Philip Durkin, principal etymologist of the OED.
"Frequently Asked Word Questions" at Merriam-Webster Online.
Papers from the 4th meeting of the European Society for the Study of English (1997) are available. The site is no longer updated.
"[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..."
By John L. Jeffcoat III.
Compiled by Akinobu Tani (Hyogo University of Teacher Education, Japan).