I helped a professor set up a wiki the other day using PBwiki. PBwiki is easy to use and offers business, academic, and personal wiki platforms. Starting a wiki is free and quick (under 90 seconds, the website claims, which is about right).
One great resource on the PBwiki is this directory of example wikis that people have created, including education-related wikis. You can access those links here. This is an excellent place to get some ideas about how to structure your own wiki.