Wikipedia's template system allows sophiticated users to greatly automate their most-common tasks — and this automation has grown so complex that the template language can now be considered a full-fledged programming language:
This led us to hypothesize that Wikimedia's template language had becoming Turing complete (the technical jargon for a full-powered programming language). We started digging and eventually were rewarded with recursive template substitution, which appears, at least at first glance, to be sufficient to implement the lambda calculus, and thereby perform as a Turing complete functional language. Hence, Wikimedia proves the Strong interpretation of Greenspun's Tenth Rule: any sufficiently advanced system will contain a functional programming language. (Which, by the way, it appears I'll have to Wikialize once I'm done this post…)
(Thanks, Alex!)
