The Corpus of American Historical English is a searchable index of word usage in American printed material from 1810 to 2009. Powerful complex searches allow you to trace the appearance and evolution of words and phrases and even specific grammatical constructions, see trends in frequency, and plenty more. Start with the 5-Minute Tour.
It has long been noted that style manuals and other usage advice frequently contain unintended examples of the usage they condemn. (This is sometimes referred to as Hartman's law or Muphry's law - an intentional misspelling of Murphy.) Starting from this observation, Joseph Williams' paper The Phenomenology of Error offers an examination of our selective attention to different types of grammatical and usage errors that goes beyond the descriptivism-prescriptivism debate. (alternate pdf link for "The Phenomenology of Error") [more inside]
Online Corpora. In linguistics, a corpus is a collection of 'real world' writing and speech designed to facilitate research into language. These 6 searchable corpora together contain more than a billion words. The Corpus of Historical American English allows you to track changes in word use from 1810 to present; the Corpus del Español goes back to the 1200s.
"I couldn't care less" vs. "I could care less"... A letter to Ann Landers in October 1960 is credited with starting the debate over "one of the great language peeves of our time." Via. [more inside]
The Michigan Corpus of Academic Spoken English is a searchable collection of almost 2 million words of transcribed spoken English from the University of Michigan, including student study groups, office hours, dissertation defenses, and campus tours. Researchers use the Michigan corpus to investigate questions about usage, like "less or fewer?" (cf. this contentious Ask Meta thread) and more general topics, like "Vague Language in Academia." Browse or search MICASE yourself.
Merriam-Webster's Ask the Editors blog is the centerpiece of the Merriam-Webster's Learner's Dictionary site. It is an excellent source of sensible advice about English language and usage. Editor-at-large Peter Sokolowski also has a Twitter feed where he highlights various interesting things about words. Finally, Merriam-Webster has started producing Ask the Editor videos, four so far, where they've tackled the subjects of i before e, classical roots, affect vs. effect and how news stories affect what words people look up online, in this case focusing on the effect of the coverage of Michael Jackson's death. Incidentally, Merriam-Webster have released their top ten words of 2009 list, which is based on what words people looked up.