Wiki is a wonderfully interesting thing. Admittedly I don't understand all the ins and outs of how it works, but it's best described strait from wiki.org as, "The simplest online database that could possibly work." The site also explains, "Wiki is a piece of server software that allows users to freely create and edit Web page content using any Web browser."
With that in mind, sites like wikipedia.org become a valuable resource for information. I find myself using it as much as Google, sometimes even over Google. Don't get me wrong, I still believe Google is the internet's Midas touch. At the end of the day it's merely a search engine that lacks the analogue touch and grace of multi-user created articles that can literally cover just about any topic in great detail. Try doing a search on something you know about on wikipedia.org and most likely you'll learn something new. Some of my favorite topics include, Ultima, Star Trek, RV, World of Warcraft, and I just found out there is no article for Airstream. Maybe I should though in my two cents.
Wiki can also be used for more detailed projects. A great example of this is the one I found last night called Memory Alpha. After Friday's Enterprise episode I wanted to find out if I remembered my Star Trek history correctly. I was right.