Per Karen Petersen:
I created a list of 101 resources for math teachers. You can see the list at http://onlinemathdegrees.org/leading-sites-for-teachers/
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For current and future math teachers everywhere, the web offers a treasure trove of resources, from advanced curricula to games and activities for teaching math concepts to students of all ages and skill levels. Many math educators are prolific bloggers, and the sheer volume of excellent math resources available freely online is astounding. This list is not intended as ranking, but as a broad and comprehensive guide to available resources, and the order is arbitrary. The sites highlighted in this collection are some of the most useful, informative, creative, and fun math resources on the web!