I've been using wikis in the classes that I teach. I set up a simple wiki with a link from the main page to pages for a class roster, syllabus, list of assignments, and topics to discuss in class. The students' first assignment is to add their names to the class roster and then create a link to their own page that they create. I know after this assignment that they understand the basic mechanics of the wiki.
During the first actual programming assignment, the students posted questions about the assignment. Some weren't clear on what was being asked, some weren't clear on grading issues, some had ideas for implementation they wanted to discuss, and some wanted to suggest changes to the requirements for the assignment. Each time I logged on, I would click on RecentChanges, and I could see which pages had changed or been added since I last logged on. I could track and participate in the discussion.
One of the requested changes to the assignment specification struck a chord with the students. I added a survey to the page where I asked them to vote, and I promised that if 12 of the 18 students voted by 5:00 pm the next day we would go with the majority opinion. The voting mechanism was simple. The students edited the page and replaced the current vote with one more in the column that they supported. Of course, I had to trust them. One student could have added 10 to the side he or she supported. In talking to the class afterwards, I found this didn't happen. The vote was honest (14 for, 0 against), and the wiki had allowed me to run this vote without any perl scripts or other programming required.
The technical aspect of setting up your own wiki is not much work. There are perl scripts provided to assist you. You can also use one of the other forms of wiki. For example, the swiki is a Squeak-based wiki. You can find out more about swikis -- to see one in action, visit the Swiki + Comanche site at Georgia Tech.
You can choose to password sites or to leave them wide open. In a way, the less security you put on a site, the less payoff there is to a hacker seeking to do damage.
To see more about creating and running a wiki you can read the book by Bo Leuf and Wiki inventor Ward Cunningham called The Wiki Way (Addison-Wesley, 2001), visit the companion web site, or visit the big wiki. Although The Wiki Way is full of interesting ideas and good advice, the companion CD has formatting errors. You can download the corrected source from the companion (wiki.org) web site. The book also provides insights into how to use wikis effectively.
You should consider the purpose of your wiki and the size of your target audience. This will help you make decisions about whether to require registration, how to bring visitors back when new content is available, whether you're concerned with generating a critical mass of content, and other meta-issues.
Wikis are an ideal setting for supporting focused projects. For example, if you are working on a grant with one or more collaborators, a wiki is a great forum for evolving your proposal. Similarly, you may need to plan a workshop with others who are separated by distance. As different speakers confirm with different organizers, the program can be shaped. You don't have to wait until an overworked webmaster has time to add the latest information. You can edit any page at any time. A small town or other community may want to use a wiki for announcements. You can take a look for announcements on upcoming talks, concerts, pancake breakfasts, etc. If your dog runs away or you are interested in selling your old bicycle, you can just post a notice. In these types of projects, visitors will return because of their interest in the underlying project. If you are running the wiki in support of your class, just the act of posting relevant announcements and responding to student concerns will be enough motivation for regular participation.
For a less focused community, you need to have a site that is initially growing to attract the participation of people who just stop by. This may require that you add up to four pages a day just to give the site a substantial feel. Some wikis support the feature that visitors can register to be notified of changes to a page or to the site. This encourages the community to visit on a regular basis. The wikis at Math Forum (including our Developers' Area Wiki) support registration -- go to Preferences and fill in the first section. By default you will get e-mail notices of changes to the site. Once the wiki is growing, people will come and people will go. Just as with any other resource, as long as it provides value, it will attract visitors.