Wikipedia.

Speaking of writing online, Wikipedia is an online encyclopedia which anyone can contribute to, either by contributing a new article or editing an old one. An absolutely fascinating idea. Have some expertise to offer? There's an entire page of requested articles to stimulate your imagination. (There are also plenty of articles that could use some correction and/or additions. But that's what's great: someone says something you think is wrong you can just dive in and change it on the spot.)

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