Concordion is an open source framework for Java that lets you turn a plain English description of a requirement into an automated test — an active specification

A nice looking tool with the right philosophy.  I particularly like in their example the simple method of providing links to manual and automated tests, as well as listing what’s out of scope.

My only criticism is that I think you eventually outgrow wikis.   You start wanting to calculate test run results, or asssociate different test data, or something, so a richer client is needed.  I haven’t solved that yet.