A "wiki" (Hawaiian for "quick") is collaborative webpage, or set of pages, usually on a specific topic, which are open and free for anyone to write or edit. They're the most radically democratic means of information-sharing. Though not without their problems and perils, they tend to be the best way to let massive numbers of people express their combined knowledge.
For example, WikiPedia is a free and collaboratively created online encyclopedia. With over 160,000 entries in the English version (and versions being created in over 40 other languages), it's actually pretty useful just as an encyclopedia. But it excels at providing links between entries, showing the connection between various ideas and people, allowing deeper context to emerge. It's a radical idea.
Indeed, it's good to remember that the very idea of the encyclopedia itself has revolutionary roots. The very first, the Encyclopedie of Diderot and d'Alembert, was an Enlightenment effort to bring together the most important knowledge and most useful innovations known to Man in a way devoid of cant, religious superstition or class prejudice - this in a day when France was ruled by a King and the Church had its own courts. In that context, the Encyclopedie was a dramatic move towards information freedom and the widest possibile dissemination of useful tools.
Whatsmore, the Encyclopedie was itself a product of collaboration, with more than 140 contributors arguing over the merits of each other's entires. Fittingly, the main effort to translate the 32-volume, 70,000-entry French book into English is itself an online collaborative one.