A few weeks ago, Brian Frank posted a link on twitter to a blog about PER that he thought many physics teachers might not know about. I wanted to point out some people that I really like reading that I suspect many people don't know about, either.
Patrick Len posts a lot of questions from his physics and astronomy classes and quizzes over on his blog. Some of the questions are peer-instruction style questions for use in class, but recently he's been posting questions related to current events in astronomy. He also posts a lot of useful materials for activities and demos that can be done in class.
Matthew Francis is a physicist now working as a science writer. I came across his blog, Galileo's Pendulum, when I was tracking down some information about a local science landmark. He writes about a wide swath of science topics, many of which aren't covered by other writers.
Dr. Drang is fairly well known in certain nerd circles. He remains anonymous online because his day job is engineering consulting for lawyers working on cases that may end up in court. He feels that keeping his online presence separate from his day job is the best choice for him. Nevertheless, he often posts really great stuff related to math, engineering and physics.