Wikipedia is an interesting experiment. In many ways.
The Wikipedizer is a web service to which one can pass a URL, and get back a list of related Wikipedia entries in XML. Example. Neat idea.
In the Danish wikipedia, there is a category about it-arkitektur (IT architecture), but I find very little on this in the English version.
Another interesting thing with Wikipedia is that the underlying engine, MediaWiki, is open sourced.