We've just launched a re-write of the Grails.org site in Grails. Previously the site was powered by Confluence (the Atlassian wiki), now in the spirit of eating ones own dog food it is a fully Grails powered site.
Ever since the Seam guys launched seamframework.org powered by Seam I felt something had to be done, before we could look them in the face again ;-)
Now its there and its more than a simple page turner, but a full featured wiki engine including versioning, diff'ing etc. and the source code is available at Codehaus. Enjoy!