Leading the charge of enterprise wikis are Atlassian Software, JotSpot, and Socialtext. JotSpot was started by the co-founders of Excite Inc. JotSpot isn't open source, but it's free to use by open-source projects. By offering features such as forms and integration with external data, JotSpot overcomes the issue of wikis being essentially limited to handling text documents.
Socialtext is based on the open-source Kwiki software. A standalone appliance with the software preinstalled is available. Atlassian's Confluence code base is composed almost exclusively of open-source libraries, and the open-source community and projects may freely use its code. Atlassian provides licensees with the source code.
On the strictly open-source and noncommercial front, there are several major players. Tikiwiki has an editorial engine for submitting, editing, and approving article submissions, as well as a workflow project-management system. TWiki can be expanded dramatically with server-side plug-in modules that allow for specific handling of functions such as calendars, spreadsheets, Really Simple Syndication, and bar codes. Zwiki offers a plug-in WYSIWYG HTML editor called Epoz that supports all the major browsers. Perspective is popular with some large companies and seems to be the wiki with which many big businesses get their feet wet.