There are a variety of wiki engines available (quite the variety, actually). Over the past year or so Mediawiki has emerged as one of if not the most popular wiki package. This is due in part to the popularity of Wikipedia and its sister projects, and partly due to the strength of their software. I note the release of version 1.5 because it has one feature of great use to those wishing to deploy it for coursework projects--Access Control Lists. Prior to version 1.5 it was cumbersome (ie you had to use hacks) to configure the wiki such that different groups of users had different permissions profiles--for example, enabling instructors to lock or delete pages, add new accounts, or move badly named pages, while not granting students those privileges. Many other wiki packages have this feature, and if the lack of it was keeping you from using Mediawiki in the past, Mediawiki 1.5 is worth consideration.