Editing
Formatting Text
MediaWiki Help pages
List of all files
Editing the sidebar
AJAX HTML->Wiki converter
When spammers hit your wiki
Bundled Extensions and Skins list - comes with MediaWiki install
EventLogging
Video
UploadWizard
MsUpload
BlockandNuke
Push
HeadScript for Google Analytics scripts
Remove 3 char search limit
Force logins to mediawiki
Disallow history reading for unlogged users
Change allowed upload file types