Free Math Drills has a large collection of sheets covering numeracy.
Kuta Software has free samples from their worksheet generator software for algebra and geometry. Good coverage of the mechanics for high-school math, some word problems too.
Math Worksheets Land wide collection of worksheets and explanation/examples covering k-12. Organized by topic or grade level.
Math.com algebra worksheet generator
Home School Math Worksheets some pre-made, some generated on the fly
MathTA.com - create printable worksheets
TuLyn.com - lots of word problems.
Super Teacher Worksheets
onlinemathlearning.com - lots of sheets covering basic addition to high-school topics
Basic Arithmetic Games
Math 131 College Algebra - college math is equivalent to University track high-school math
HippoCampus - videos, presentations, worked examples, interactive simulations and test prep for middle school and college
Interactive Web Sites
Online plotting tool - seems to be able to plot anything and you can download the plot in various formats
Making Change Game
Math Frog A site from the University of Waterloo filled with fun resources and interactive math games targeted to grades 4, 5, 6, including an interactive geoboard.
National Library of Virtual Manipultives from Utah State University is a set of Java Web apps that provide manipulatives on the computers. Pattern blocks, geoboards, charting and graphing, transformations, number problems and much more. No lesson plans, but lots of raw tools to use.
Shodor's Interactivate website resources for all math strands. They all seem to be written in java, so should work on all operating systems and browsers.
Interactivate site National Council of Teachers of Mathematics free resource page. Over 100 online activities, 500 lesson plans and web links for K-12.
Coolmath.com An amusement park of math and more. Tons of games for all strands.
Math Playground Games, puzzles, word problems, videos, timed flashcards
Learn Alberta The site's been recently redesigned so some of the great animated lessons are hard to find. It's now all accessed via a search system, but once you find what you want, you can build a workspace to collect it.
Brainpop - Animated Educational Site
calculus.org: THE CALCULUS PAGE Links to all sorts of calculus resources, including applets, animations, tutorials, sample tests, questions, etc.
Math Motivation Learning the value of Mathematics in your Life and in your Career
ABCya has a bunch of math and other games for the lower grades
eWorkshop Ontario Ministry of Education website with resource for Primary and Junior grades (K-6). It contains modules for Literacy and Numeracy. As opposed to most sites, these modules are geared toward teaching the teacher. Among other things it includes videos of the lessons being taught to real students.
Ontario Ministry of Education - Math This contains the curriculum guide, exemplars, and the reports Early Math Strategy, Leading Math Success, and Teaching and Learning Mathematics
Math Forum This site from Drexel University contains forums for talking to other teachers and experts in the field, Lesson Plans, Problems of Week. Memberships start at $25 per year. There is some content that's accessible for free, notably the forums.
Oswego School District - Regents Exam Prep Center There are short on-line lessons (non interactive), practice sheet, and resources for teachers (for example, creating activity stations).
Ontario Association for Mathematics Educators Resources for mosly high-school math, although there's some targeted to intermediate, including extra resources for Geometer`s Sketchpad software
Mrs. Dell's Geometry Unit A 13 lesson geometry unit with mini-lessons and games and activities for each.
Peel Schools Math Online Another collection of pointers to web resources. They are sorted by strands and objects of the Ontario Curriculum
Cirriculum Services Canada
Understanding Math (very expensive) software with hard copy exercises available on the website.
New Zealand Maths A NZ Ministry of Education site with lesson plans for all strand of math.
University of Regina's Math Central A different sort of resource. There's sections on math beyond school for real world applications, and math with a human face.There's also a teachers bulletin board and a place you can ask questions. There's also a problem of the month, but it looks like it's geared toward high school or university.
Reviewed websites for teachers
Platonic Realms Math moments, jokes, historical notes, links. Cheap downloads (couple of bucks each)
Computational Procedures Math Challenges for Families
PBS Teachers Lesson strategies for all strands
Math Is Fun - Homework Help Good site for review and practice.
Wolfram Mathworld the web's most extensive mathematics resource, or so it claims, and it's probably right. If you've forgotten something about anything in math, you can find it here.
Windows into Elementary Mathematics Prominent Mathematicians discuss topic relevant to Elementary Mathematics
Purplemath Helping students gain understanding and self-confidence in algebra.
OnlineMathLearning has videos and worksheets for all levels and subjects in math.
Jump Math - resources to promote numeracy in society
You CAN Do the Cube - Rubics cube lesson plans