Plugins are mainly user-contributed add-ons that enhance and extend TWiki features and capabilities. A limited number of plugins are included in the core TWiki distribution - and most those can be removed - while the rest are optional, available from TWiki:Plugins/WebHome.
Here is a list of the plugins currently installed on this TWiki site:
SpreadSheetPlugin(any TWiki, $Rev: 16273 (22 Jan 2008) $): Add spreadsheet calculation like "$SUM( $ABOVE() )" to TWiki tables and other topic text
BlackListPlugin(2013-03-22, $Rev: 25492 (2013-03-22) $): Utility to keep malicious users away from a public TWiki site
BreadCrumbsPlugin(v2.01, $Rev: 0 (01 Sep 2008) $): A flexible way to display breadcrumbs navigation
CommentPlugin(TWiki-4, $Rev: 15776 (22 Jan 2008) $): Allows users to quickly post comments to a page without an edit/preview/save cycle