Wiki is style of a site that allows the readers to correct the existing articles and to load new articles. Also, wiki is generic name for such a site. Some wikis have restriction, allowing the editing to only registered users (some times only for some of articles). Many wikis use English as main working language; some of them allow also texts in other languages.
In particular, there exist the following wikis:
http://mizugadro.mydns.jp/t , TORI (Since March 2011 to February 2013, it had been available at tori.ils.uec.ac.jp/TORI ) Tools for Outstanding Research and Investigation. This wiki. Articles about Science in the extremely strict meaning, and also the illustrations (that may be not scientific, but should have some religious or artistic sense).
http://wikimediafoundation.org Wikimedia, biggest worldwide cluster of wikis that tries to collect all important human knowledge. Good for navigation over its wikis.
http://en.wikipedia.org Wikipedia, the biggest (and, as result, most disordered) encyclopedia in the world. Has versions in many languages. Good place to begin a new search.
http://math.eretrandre.org/hyperops_wiki Site by Bo, dedicated to superfunctions and other hyperoperations. The site may be continuation of the the http://math.eretrandre.org/tetrationforum/index.php (dedicated to the tetration and similar functions).
http://www.proofwiki.org Site dedicated to mathematics. Good place to get the most refined definitions and theorems in very "dry" form (no speculations around definitions are allowed), but articles are still oriented to the human reading.
http://www.wikiproofs.de Site dedicated to mathematics in the sense of formal logic: the algorithmic proofs are suggested in the computer-oriented form. Good place to get analysis of the fundamentals of arithmetics, beginning since "1+1=2", but not only arithmetics.
http://rosettacode.org/wiki/Rosetta_Code programming chrestomathy site. The idea is to present solutions to the same task in as many different languages as possible, to demonstrate how languages are similar and different, and to aid a person with a grounding in one approach to a problem in learning another. The LOGO is dark irregular spot.
http://wikisource.org Collection of already published documents. Good place to get some original texts, sometimes even about mathematics and physics, but the criteria of significance of documents are strange; in the Russian section, they prefer texts by members of the Union of the Soviet Writers (Союз советский писателей) and other soviet propaganda.
There was site www.wikilivres.info Collection of texts that are allowed for free distribution in Canada, but since 2012, the content disappeared,
http://wikisophia.org Wiki with advanced support of pseudographics. Good source of texts with highly-compressible graphical indormatin. (Music, chess, perturbative diagrams, structured chemical formulas, etc.)
http://freakopedia.ru Freakopedia, Фрикопедия. Enciclopedia of pseudoscience, In Russian. Still not so many articles; the English version seems to be absent. Good place to get info about the most known frauds and pseudo–scientists.
http://cyclowiki.org Russian wiki about History of the USSR and other related topics. In Russian. Good place to get info about the USSR, that was censored–out from the Russian WIkipedia by the pro–Soviet participants.
http://wiki.ru-vederko.ru Общество Синих Ведерок. (In Russian.) Information about license plates of cars of the aggressive rule-breakers (and not only) There is bud about loading of the LOGO.
http://hr2.memo.ru/wiki/Хроника_текущих_событий Chronicle of current events, cite with documents of soviet samizdat; mainly in Russian. Good place to get primary information about crimes of soviet veterans.
http://www.mediawiki.org Site about installation and support of wiki. Good place to begin with, aiming to create a new wiki.
There are many other wikies in the world; their amount grows fastly. Due to the hard set of the axioms formulated at the Main Page, TORI is not supposed to compete with any others wikis. In many cases, the link to the article in another wiki (especially if from the list above), perhaps, with a short summary (for example, the preamble with few references), is better than repetition of the similar text in TORI.