Erfurt Wiki is a WikiWikiWeb implementation in the PHP scripting language and mainly contained in one script file. It is PublicDomain software. 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).
There are also hundreds of other WikiWikiWeb implementations available. You can find out about them on the large WardsWiki:WikiEngines list or using the WikiFeatures:WikiEngine comparision table.