What websites should be whitelisted on school content filters?

Wesley Fryer, Moving at the Speed of Creativity, Jan 13, 2011
Commentary by Stephen Downes

The answer to the question in the title may be an endless list, but I have to applaud the initiative as an attempt to stem the tide of blanket blocking of whole swathes of websites in schools. Enter your suggestions of sites that should be whitelisted in this form to contribute to the project.
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