The penultimate Thing, and I seem to have a mental block! I'm not really sure what I can say about wikis that hasn't been covered in the Cam23 2.0 post for this Thing. My experience of them, aside from regular (probably too regular) use of Wikipedia is very slight. I do remember using a wiki to edit the library webpages a while back, and having a bit of trouble navigating to the pages I wanted to change. You had to reach pages by inputting the correct pathway, and quite often I'd be told that a particular page didn't exist, when I knew it was there somewhere! Our web officer has since moved us to WCMS, and I find this much easier to use, since you move through the webpages exactly as you would on the live site, and then just choose the page you want to edit. Looking at Phil Bradley's weblog for inspiration as to what else I could possibly say, I found that he too seems to have talked very little about wikis in the past few years - perhaps they really are diminishing in importance. In 2006, however, he did provide a link to this interesting article: What is a Wiki ... which gives advice on using Wikis for projects and covers some of the advantages and disadvantages to using them.
A glance at a site like WikiIndex seems to undermine the idea that wikis are on their way out. It has a long list of recently added wikis, from the serious to the slightly strange!