GeoGebra - www.geogebra.org resources for all grade levels
Math Open Reference - free interactive math textbook that covers HS geometry and plans to expand
Ask Eric Lesson Plans -- Some very nice math lesson plans for all grade levels
Math Lessons and Courses -- An absolutely amazing collection of math lessons and courses...must see
Mathematics Lesson Plans -- Yup -- even more math lesson plans
Helping Your Child Learn Math -- Not a collection of lesson plans but excellent information for teachers to give to parents
SMILE Program Math Index - more than 200 single-concept lessons for grades 1-12. All lessons include templates, data sheets or graphics, plus instructions and expected outcomes
SOS MATHematics - includes well written lessons for teachers
Glossary of Mathematical Mistakes - makes a great starting point for attention-grabbing lessons
Math Tools - sponsored by Math Forum - starts at Pre-K
MiddleSchool.Net:Calculators - has all types of calculators
Math: Here, There & Everywhere - includes great activities by US Dept of Ed - great for classroom and home
Balanced Assessment - over 300 math assessment tasks developed at Harvard Graduate School of Education - topics include averages, sums, detective stories, triangles and volumes, to name a few.
Figure This! - English and Spanish math puzzles to help students master high-level concepts - for secondary students.
MIT - http://ocw.mit.edu/high-school - free onlilne courseware in math, science & humanities