For a kid who grew up with Hypercard and a rabid addiction to knowledge, Wikipedia is a dream come true. I believe it is certainly the most groundbreaking application of the Internet to date.
Thanks to SlimVirgin for creating this lovely user page template and thus precipitating my first experiment with the transclusion features of Wikipedia. I've been flying the "true transclusion for HTML!" flag since 1995 and was sorely disappointed by the implementation of frames and iframes. Working inside MediaWiki seems a lot more fun and productive. (Also thanks to Bignole for helping me stumble into SlimVirgin.)