Extensions

You can see what extensions have been enabled on your wiki at (with your wiki name instead of "example"):

https://example.miraheze.org/wiki/Special:Version

The list of already installed extensions on Miraheze (this does not mean that you can't request new ones!):
 * https://github.com/miraheze/mediawiki/tree/REL1_28/extensions

Changes to extensions are documented on:

https://phabricator.miraheze.org/

Extensions FAQ
Mediawiki
 * https://www.mediawiki.org/wiki/Extensions_FAQ

Extensions enabled by default

 * Echo
 * NativeSvgHandler
 * Scribunto
 * AbuseFilter
 * AntiSpoof
 * Babel
 * CategoryTree
 * CentralAuth
 * CheckUser
 * Cite
 * CiteThisPage
 * CodeEditor
 * ConfirmEdit
 * ConfirmEdit/ReCaptchaNoCaptcha
 * Disambiguator
 * Gadgets
 * GlobalBlocking
 * GlobalCssJs
 * InputBox
 * Interwiki
 * LocalisationUpdate
 * ManageWiki
 * MassMessage
 * Math
 * MirahezeMagic
 * Nuke
 * OATHAuth
 * OAuth
 * ParserFunctions
 * Poem
 * RenameUser
 * SiteMatrix
 * Timeline
 * Thanks
 * TitleBlacklist
 * WikiEditor
 * CLDR
 * UrlShortner
 * MobileFrontend

Extension List
Last Updated On 2017-04-10
 * AccessControl
 * AddHTMLMetaandTitle
 * AdminLinks
 * AJAXPoll
 * ArticleFeedbackV5
 * ArticleRatings
 * AuthorProtect
 * AutoCreateCategoryPages
 * AutomaticBoardWelcome
 * BetaFeatures
 * BlogPage
 * MSCalender
 * CharInsert
 * CollabsibleVector
 * Comments
 * ContactPage
 * CookieWarning
 * CreatePage
 * CreateWiki
 * CSS
 * CustomNavBlocks
 * DPLForum
 * DynamicPageList
 * DynamicPageList3
 * Editcount
 * EducationProgranm
 * FancyBoxThumbs
 * FlaggedRevs
 * Flow
 * FeaturedFeeds
 * HeaderTabs
 * HideSection
 * HighlightLinksInCategory
 * ImageMap
 * JavascriptSlideshow
 * Josa
 * LabledSectionTransclusion
 * LinkSuggest
 * Loops
 * Maps
 * MassEditRegex
 * MediaWikiChat
 * MobileFrontend
 * MsUpload
 * MsLinks
 * MsCatSelect
 * MultimediaViewer
 * MultiBoilerPlate
 * NewestPages
 * Newsletter
 * NewUserMessage
 * NewsUsersList
 * NoTitle
 * PageTitle
 * PDFEmbed
 * PDFHandler
 * Poll
 * Popups
 * ProtectSite
 * Quiz
 * RandomSelection
 * RelatedArticles
 * ReplaceText
 * RSS
 * SandboxLinks
 * ScratchBlocks
 * SimpleToolTip
 * SiteScout
 * SocialProfile
 * SubPageFun
 * SubPageList3
 * SyntaxHighlight
 * Tabber
 * Tabs
 * Translate
 * TimedMediaHandler
 * TitleKey
 * TorBlock
 * Variables
 * VisualEditor
 * WebChat
 * Widgets
 * WikiBase
 * WikiForum Currently disabled due to issues with Captcha. You may request and it will be enabled at a later time when it is fixed
 * WikiHiero
 * WikiLove
 * WikiTextLoggedInOut
 * YouTube

Declined extensions (<1.28)
Those extensions which are not available and a brief reason.
 * BlueSpice - security risk
 * Configure - working on a similar extension and doesn't work with MediaWiki 1.28
 * ConfirmAccount - incompatible with CentralAuth
 * Contribution Scores - not working
 * CrossReference - doesn't work on MediaWiki 1.28
 * DeleteBatch - because of technical limitations, see T956
 * DNSLookup - invades user privacy
 * DynamicPageList - flagged as unmaintained
 * EditAccount - incompatible with CentralAuth
 * EmbedAnything - security risk
 * EmbedVideo - security risk
 * ExternalLinks - security risk, see T1555
 * MiniInvite - security risk
 * MultiUpload - flagged as unmaintained
 * SecurePoll - reveals private information
 * Semantic MediaWiki - alters MediaWiki too much.
 * Semantic Text Annotator - requires Semantic MediaWiki
 * Site settings - working on a similar extension
 * Tasks_Extension - flagged as unstable
 * UploadWizard - Extensions to specific to Wikimedia use. Unsuitable for independent wiki use.
 * UserMerge - incompatible with CentralAuth
 * Username - invades user privacy
 * WikiBanner - we don't accept ads

Disclaimer
Some extensions enabled by default cannot be managed/used by individual local communities at this time. These include CheckUser, RenameUser and GlobalBlocking (which are currently limited to global Stewards) and some extensions only have a function on Meta such as CreateWiki. Also note that the ManageWiki extension can only current be used by wikicreators, syadmins and stewards.