Verbatim magazine, edited by Erin McKeen:
http://www.verbatimmag.com

Phrase Finder, which sends weekly offerings to readers:
http://www.phrases.org.uk/index.html

Ask Oxford and Oxford Online:
http://www.askoxford.com

New York's Authors Guild, an organization for published authors:
http://www.authorsguild.org

Bartleby.com, great books online:
http://www.bartleby.com/61

Histories of the English language:
http://ebbs.english.vt.edu/hel/hel.html
http://www.wordorigins.org/histeng.htm

World Wide Words, by Michael Quinion:
http://www.worldwidewords.org

Free online encyclopedia:
http://en.wikipedia.org/wiki/Englishlanguage

WordNet, an online lexical reference system:
http://wordnet.princeton.edu

Merrian-Webster Online:
http://www.m-w.com/help/faq/history.htm

Oxford's ELT Journal, for scholarly language articles:
http://eltj.oupjournals.org

EL Gazette, an English language journal:
http://www.elgazette.com

A few words that differ between English and American English:
http://www.effingpot.com/index.shtml
http://esl.about.com/library/weekly/aa110698.htm