MoinMoin is an advanced, easy to use and extensible WikiEngine with a large community of users. Said in a few words, it is about collaboration on easily editable web pages. MoinMoin is Free Software licensed under the GNU GPL.
If you want to learn more, first read about WikiWikiWeb, then about WhyWikiWorks and the WikiNature.
If you want to play with it, please use the WikiSandBox.
MoinMoinFeatures documents why you really want to use MoinMoin rather than another wiki engine.
MoinMoinScreenShots shows how it looks like. You can also browse this wiki or visit some other MoinMoinWikis.
