These blogs are great sources for original and thoughtful math activities.
Fawn Nguyen's blog
Includes many subtopics, such as her blog on Math Talks in Elementary, Middle and High School
The Math Lab @ Room 27
A new blog with some great tasks and activities for K-5.
This interactive site contains more than 101 questions to perplex, intrigue, and generate mathematical situations for K-12.
Questioning My Metacognition
A great source for a variety of problems, including 3-Act problems for Elementary.
Challenging math problems worth solving for grades K–12.
A great source of innovative thoughtful problems and teaching ideas for Elementary teachers.
Steve Wyborney's Blog (I'm on a learning mission)
Steve's blog contains a wealth of resources for the K–8 classroom: Topics range from number to geometry, and include visual thinking, estimation, and connections between domains.
Build Math Minds / The Recovering Traditionalist
Christina Tondevold blogs about teaching math with a focus on Elementary math.
Dan Meyer's blog: dy/dan
Dan's blog is a wide-ranging melange of philosophy and practice, primarily aimed at Middle and High School teachers, but includes some Elementary problems as well.
Geoff Krall's blog: emergentmath
This blog is dedicated to brainstorming interesting and dynamic math problems and projects with a Project/Problem Based Learning (PBL) models of instruction theme. Middle and High School.
Would You Rather?
Problems that as students to choose their own path and justify it. Mostly for Middle and High School.
Don Stewards blog
Tasks for teaching math to 11 to 16 year olds, Middle and High School.
Becoming the Math Teacher You Wish You'd Had
Tracy Zager writes about problem solving and imaginative thinking and teaching math with understanding.
Steve Miller's Math Riddles
A wonderful repository of riddles for kids of all ages, together with information on using riddles in the classroom.
Rachel Lambert's collection of math + disability resources.
Andrew Stadel's blog with estimation problems for Middle School - great for building number sense.
Robert Kaplinsky's Site
Contains a variety of problem-based lessons and performance tasks, K–12.
Mr. Orr is a Geek.com
A variety of problems for Middle School including 3-Act Problems.
Another blog from Dan Meyer for Middle and High School graphing quickies.
Henri Piccioto's Math Education Blog
Henri is a long time innovative High School math teacher who blogs about a variety of topics, especially Geometry and classroom innovation.
Essays about Secondary Standards
Essays about Middle and High School standards and the math content behind them.
Teaching to the Beat of a Different Drummer
Brian Bushart's blog about Math and Curriculum, including his Numberless word problems for all grades with a focus on Elementary.
The Student Centered Mathematics Classroom
Dynamic online games and tools for K–12.
The Learning Kaleidoscope
Andrew Gael's blog exploring the intersection of Special Education, Math Education, and Disability Rights.