The wikiproxy dynamically links "Proper Noun Phrases" in BBC News Online stories, typically names of people or organisations, to their appropriate entries in wikipedia. Since the original version, the same idea has been deployed on Theyworkforyou.com, turned into a Greasemonkey script, and more.
However, this is not the only use. There a number of interesting applications that derive from not only the knowledge, but the meta-knowledge that wikipedia captures.
I will outline how some of these applications work, what others there might be, and finish with some ideas that a wikipedia API could deliver, way beyond a humble (!) encyclopedia.