Plugins are just my thoughts on how to add new function to wiki. I do agree that the simplicity of wiki is what draws me to it, so that would not be desired to be spoiled.
examples: http://www.dowtech.net/index.php/CalendarPlugin http://www.dowtech.net/index.php/WikiPlugin
twiki: http://twiki.sourceforge.net/cgi-bin/view/Plugins/WebIndex [inter wiki plug-ins]
php: [php plugins]
MoinMoin: MoinMoin:MoinMoinExtensions
See also ExtensibleWikis