It's a collaborative space developed by Mark Guzdial and others in an open source effort, based on the WikiWiki by Ward Cunningham. A Swiki is a Wiki in Squeak. Basically, any page can be edited by anyone, and anyone can create new pages.
We've created a wide variety of Swikis, from those that require passwords (PSwiki, AuthorizedSwiki, AuthorizedWriteSwiki), to those that allow embedded computation (ActiveSwiki), to those that render and FTP pages to external websites (RenderingSwiki). (This website is created with a RenderingSwiki.)
For more on Swiki, see http://pbl.cc.gatech.edu:8080/myswiki.1