The Illustrative Math Project gives illustrations of the new CCSS. Many of the standards have nice, rich, high quality example problems.
Khan Academy Tons of helpful little videos on math algorithms and a nice coaching program.
Wolfram Alpha is more than a search engine. It gives you access to the world's facts and data and calculates answers across a range of topics.
Shodor Applications, games, simulations and math tools.
Desmos Graph functions, plot tables of data, evaluate equations, explore transformations, and much more – for free!
The Teaching Channel has nice videos and lessons on all subjects. The math lessons look good and tend to focus on concepts and reasoning.
Dan Meyer writes about the use of technology in the math classroom as well as writing and implementing tasks that help students to become patient problem solvers.
Great Maths Teaching Ideas is a math education blog that is full of current math ed news and great teaching ideas for various math concepts. We believe that William's philosophy matches up with the philosophy of the NCTM and CCSS. Follow his blog for ideas and resources that you can put to use in your classroom.
Emergentmath A blog about teaching in an engaging and effective manner with a problem based learning focus.
Robert Kaplinsky If you like yummymath you should check out Robert's real world math activities.
DeltaScape Nice blog about math ed and issues relating to teaching and learning.