Robert Kaplinsky's Problem-based Search Engine
youcubed.org is the brainchild of professor Jo Boaler at Stanford University. Jo and her students in the mathematics education program at Stanford have put together an amazing site with lots of incredible tasks that support growth mindset in mathematics. There is so much here to investigate! Check out the Week of Inspirational Math (week 2 now includes lessons for K-2 students), the online courses for students, teachers, and parents, and lots of rich tasks and research articles.
Numberless Word Problems are designed to provide scaffolding that allows students the opportunity to develop a better understanding of the underlying structure of word problems.
Geoboard is a tool for exploring a variety of mathematical topics introduced in the elementary and middle grades–a great tool for developing an understanding of fractions!
Dan Meyer's Three Act Tasks
A three act task is a rich open task with minimal scaffolding. In the first act the problem is introduced visually, with as little introduction as possible. The second act is where students define the problem, determine the information they need to solve it, and work through a solution. The third act is where the students come to an agreement about the solution.
Kaplinsky also provides problem-based lessons in the form of Three Act Tasks. Students answer engaging questions such as, "How big is the world's largest deliverable pizza?"
GFletchy does the same--his tasks are listed by grade level and standard.
Catherine Castillo, Mike Wiernicki, and Kendra Lomax host sites with more great tasks.
Yet another fantastic tool from Steve Wyborney. The Maze Hundreds Chart is a fantastic number talk tool Use for exploring the hundreds chart, one more or less than, ten more or less than...Be sure to use via powerpoint--you can download the app on your ipad.