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Curriki is a collaborative website containing tons of curriculum for math, language arts, science, and social studies.
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If you're a giver, you can contribute lessons or units you've created to Curriki. The site definitely depends on the givers out there to amass the wealth of resources they have.
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If you're more of a taker, you'll find plenty of good quality lessons and units here. I used parts of several units from Curriki this year for some history lessons on the spread of Islam. I was able to download the background info I needed to teach some lessons, and also some great individualized projects to go with it that were created by a contributing teacher. They were so good I wrote him a thank-you note/comment on the site!
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If you're both a giver and a taker, then you'll love the collaboration aspect of this site. It's possible for you to work with one or more teachers in your content area to contribute materials and create a joint lesson or unit!
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Becoming a member of Curriki is simple - just the usual name and email information. I can tell you from personal experience they almost never bug you with info or updates. I've gotten maybe 2 emails from them since I first joined last August.
I can tell you that the site is really easy to navigate, and more new materials are added all the time!