A Wiki is an easy-to-use collaborative editing environment that often uses a revision control control system to keep tabs on changes made to documents. I was going to use one to keep track of ideas and essay sketches I was writing for my university honors thesis, so I wanted it to be simple, to have a design that was easy to change, and to support user authentication. After reviewing several such projects, I finally settled on TWiki.
TWiki offers a useful solution for sites in which there are different groups working independently who wish to have common or overlapping areas as well. With permissions set up correctly, it offers access control that, although somewhat defeating the "anyone should be able to contribute" purpose, means that it can also be used for less public material, making some Webs invisible to visitors and others read-only, etc.
TWiki is easy to theme (offering several skins that are easily adapted to my requirements), easy to set up, and easy to use. With a small amount of assistance, most of the non-technical people I introduced to it were able to participate in group work and discussion through it. The number of plugins available is also impressive, with a range that includes calendars, pretty-printers, spell checkers, etc. There is also an RSS feed available for each Web. The performance is good, too; it runs speedily enough on a low-end Celeron, without any complaints. Auto-notification is a useful addition to the feature list, though it would be nice if it were enabled by default (instead of requiring the user to specifically add a cron job for it). The configuration of user authentication was a bit fiddly, but the documentation is very good and easy to follow. Despite these issues, it's a fun, easy-to-use tool that is extremely flexible.