First, let's be clear...a forum is not a wiki.  Some definitions---

According to wikipedia:

A wiki is a website which allows its users to add, modify, or delete its content via a web browser using a simplified markup language or a rich-text editor.

 

A forum is an online discussion site where people can hold conversations in the form of posted messages.

So the intended purpose of a wiki is to collaborate on editing a single piece of content, whereas a forum is intended to host an ongoing, back and forth conversation (i.e., a chain of separate pieces of content).

However, there are many aspects of a forum platform that can be made to function "like" a wiki.  This might be useful in a scenario where the owner wants to consolidate onto a single platform, or the members don't have the technical know-how to use a wiki.  A forum makes an excellent knowledge base.

Key characteristics of a wiki:

• Can be edited by multiple people
• Creates an enduring knowledge base
• Can be added to over time
• Ability to cross-link to other posts

Most forum platforms allow for all of the above (Hoop.la does).

These are some ideas for setting up forums to include a "wiki-like" area:

• Some wikis allow the general public to edit; in this scenario it would be best to restrict editing to a specific group in order to avoid spam.
• In Hoop.la, you'd give your "wiki" members the "Manage All Topics" permission using permission groups.  That would allow them to edit, move, and delete topics (lots of power, so be certain about your trust level here).
• Use the Topic title as the title of the article, and post the initial body content. You can then immediately close comments if you wish, and invite members with permission to edit the body.
• Once the body has been edited, a note will appear at the bottom of the post, saying "last edited by username, time."  It will only show the most recent editor.
• The entire forum (and Hoop.la site) is searchable, so articles of interest can be easily located.
• Hoop.la allows you to include embedded video and photos inside the forum posts, for a richer "wiki" page.
• If you don't want to give edit permission to members, you can still post a proposed article as a forum topic and invite peer groups to comment with their suggested edits in the topic. 

 Are you using your forums as a knowledge base? Do you have any special tips?