There are literally dozens of different wiki platforms out there all of which fulfill a different set needs. For those dizzied by the array of choices, the Wiki Choice Wizard from WikiMatrix looks like a good place to start. It asks you a few questions about your needs--e.g. hosted or installable, WYSIWYG or otherwise, and so on--and then narrows your options to a few that match your needs and lets you compare them in a grid.