Mrs. Ellis

Merriam Webster Dictionary

Motivated Grammar
("Prescriptivism Must Die!")

Englishchick

My favorite.
http://www.merriam-webster.com/

Heart-felt debates about the fate of the language. Recent topic: do you say "I'm good" or "I'm well" when someone asks, "How are you?"
(Note: "prescriptivism": a rude habit of telling people what they ought to do).
http://motivatedgrammar.wordpress.com/

Short, clear explanations of grammar basics by a professional editor who's in tune with high schoolers and college kids. WARNING: this site is part of a personal website that promotes the author's professional editing services. I include it on this page because explanations are short, clear, and user-friendly.
www.englishchick.com/grammar/ggrem.htm