This site has pretty much all of the formulas you would ever need for all disciplines of math. Useful resource to give to students to do homework or to study with.
This website has useful links to resources that teachers can use. Similar to diigo in that you can bookmark your own links that you like on the page. There are so many links.
Interesting article about an Experimental school where there are no classes and no teachers. Students work on projects on their own schedule, with advisers just making sure they don't fall behind.
This is Sam Shah's blog. He's a math teacher in New York, and while often not as entertaining on his blog as some others, tends to speak fairly frankly and openly about his activities in the classroom. He tweets as @samjshah.
This is Karl Fisch's blog for his algebra class this year. It's interesting to watch, if only for the ideas he presents and the activities involved. I believe he also does SBG, and tweets as @karlfisch.
This is Shawn Cornally's blog. He's a high school science/math teacher, and a big advocate of Standards Based Grading (SBG). He tweets as @ThinkThankThunk.
This is Kate Nowak's blog. She is a high school math teacher, and has been blogging for a while. She's often an amusing writer, and tweets as @k8nowak.