| I use wikidot, but I admit that I started mine before I heard about obsidian portal and now I am sad. I could move it over, but the wiki I have suits me fine and ... I'm too lazy.
BTW: As a guy who loves reading other campaign wikis, a BIG TIP: Put an introduction page or section there for guests; like a "Start here if you're new". It'll help people visiting and more importantly, it gives your players a place to look up the most relevant info.
I use a "player's guide" type section called "Adventuring" and then I'm writing down the stories of the different campaigns in chronological order to help tell the story.
Its too easy to make a wiki that is full of nothing information to others. | 
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