ErfurtWiki is a WikiWikiWeb implementation in the PHP scripting language and mainly contained in one script file. It is PublicDomain (you are free). It was implemented more like a library and thus allows very quick integration into existing web sites, also because it does not enforce a specific page layout or requires you to choose from a few factory page themes/skins.

See the FeatureList or SoftwareNotes for what it can do. Or find out more about it at ErfurtWiki: or http://ewiki.berlios.de/ (where you could also get your own, using the very simple WebInstaller). Also see the OnlineDocumentation for discussion of some advanced setup tasks.

There are also hundreds of other implementations available. You can find out about them on the large WardsWiki:WikiEngines list or using the WikiFeatures:WikiEngine comparision table (though still incomplete).