Wikipedia
WikkaWiki
WikkaWiki (often shortened as Wikka) is a free, lightweight, and standards-compliant wiki engine. Written in PHP, it uses MySQL to store pages. WikkaWiki is a fork of Wakka Wiki to which a number of new features have been added. It is designed for speed, fine-grained access control, extensibility, and security, and is released under the GNU General Public License.