Wiki is the name for a software that shows you a web page that everyone can edit. There are many different wikis, the most famous one is mediawiki which is used to run wikipedia. tilswiki is yet another wiki software. In contrast to most other wikis it does not require you to click an edit button and then to save your changes - rather it is always in edit mode. So you can click in the text on any tilswiki page and edit it immediately, for example the page you are currently reading.
It uses the contentEditable attribute, and only works reliably on Firefox 3 right now.
tilswiki is open source, under the AGPL. Notice the A - source code changes must be shared, even if the modified version is only accessible through a hosted a web app.
An instance of tilswiki is hosted at http://wiki.tils.net/
Editing only works on Firefox 3 reliably. Viewing works on other browsers, could look nicer though (needs a bit more work).
http://wiki.tils.net/about