Wiki - a website that allows anyone to edit any page using very simple markup (like HTML, only simpler). They have easy ways to create new pages, and links between them. There are more details about Wiki's in Wikipedia, (a multilingual project to create a complete and accurate free content encyclopedia, using a Wiki).
Scrum - a way to manage projects - typically software development projects, but almost any project can be managed with Scrum. It is an agile process: "people over process and tools" and focuses on delivering real stuff that the business wants, and frequently. Wikipedia also has an entry for Scrum.
ScrumWiki - a Wiki for Scrum. It has all the regular Wiki features, plus it adds the functionality to make managing Tasks, Teams, Sprints and Product Backlogs easier.
ScrumWiki is not an attempt to re-invent the Excel wheel. If you want tabulated data, and all the other great things Excel does, this won't be for you. (Although Google Spreadsheets have some nice online sharing and revision control features you may want to check out). If you are looking for a simple, agile approach to project management and a way to capture requirements, changes and documentation together in one easy system, then this might be for you.
We used to have a running demo here. But too many people were trying to abuse it as a public wiki, and we ran out of time to support it. The software is not under active development, but you are welcome to grab a copy and play.
http://groups.yahoo.com/group/scrumwikidev/
Note: You do not need a Yahoo! account to subscribe to the mailing list. Just send an email to scrumwikidev-subscribe@yahoogroups.com.
Our "official" historic releases are listed below. However Andreas Ebbert-Karroum has uploaded a much more recent version (Dec 2007 - about four years since 0.2!!) with patches applied etc. See here for the announcement (please read notes about testing) and you can download his version from here. You'll want "scrumwiki-0.3.zip"