Cut the Knot – A tremendous resource for math ideas, lessons, and visualizations.
dy/dan – Dan Meyer’s math blog showcases some great ideas in math pedagogy, especially for the middle and high school levels.
Emergent Math – A showcase of some of the best lessons for 6th-12th grade that the internet has to offer. Probably the internet’s best chance to replace textbooks. Totally free.
Let’s Play Math! – A play and math infused website overflowing with great ideas, quote, games, and resources for the younger set.
Living Math – Homeschooling and have questions on how to get your kids on the right track with math? This is a great place to start. A great community to answer your questions, and tons of resources, including a tremendous book list.