Personal Wikis?

Anyone have a wiki that they use for their own personal use? I'm thinking of getting one to organize documentation and step-by-step instructions for various tasks I have at work, and for some other stuffs. Anyway, that's besides the point. I'm wondering if you all have any good, free (or cheap at least), third party hosted wikis to recommend? I don't want to run my own server.
GitHub repositories have built-in support for wikis, and a variety of wiki formats are supperted, such as mediawiki and Github Flavored MarkDown.
Trac :)
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