Where can I find a list of available OS X "Extensions"?
You can search the Mac App Store for apps supporting a specific extension type. For example, searching for appex:com.apple.share-services will show apps that support the share extension. Not all macOS apps are distributed through the Mac App Store, so this isn't an exhaustive list.

Common Mac Extension Types are listed below:


Action: com.apple.ui-services
Audio Unit: com.apple.AudioUnit-UI
Content Blocker: com.apple.Safari.content-blocker
Document Provider: com.apple.fileprovider-ui
Finder Sync: com.apple.FinderSync
Photo Editing: com.apple.photo-editing
Share: com.apple.share-services
Today: com.apple.widget-extension