A TagWiki is a natural generalization of the wiki concept.
Each WikiWord is automatically made a tag, even the Title. This technique makes a WikiEngine much more powerful, because it generates local and global links at the same time, without more user-effort.
A more detailed description of a TagWiki is described on the Meatball TwinPage.
Because WardsWiki is probably the wiki with the largest community of engaged WikiEngine developers, he put it on the list: WikiEnginesWanted. What about a TagWikiContest starting at 2007/06/01 14:24?
TwinPages: http://www.usemod.com/cgi-bin/mb.pl?TagWiki