Dan Meyer's Blog-- Dan Meyer's posts regularly and in thought provoking ways about teaching practices and ways to improve. He is humorous and highlights other comments and views as well.
Dan Meyer's 3 Acts-- So Dan Meyers gets two spots here, because his stuff is great. This site has video prompted math lessons in three acts. They pose an engaging, visual question to be answered.
Questioning My Metacognition--Similar to Dan Meyer's 3 acts, for elementary.
Mathematics Assessment Project--Fantastically detailed, task-based lesson plans for grades 6th-High School