is there a list of plugins that are in use on WordPress.com somewhere?
It would be very interesting to see how they have implemented specific widgets and functionality.
closed as off-topic by Pieter Goosen, toscho♦ Jul 9 '14 at 1:25
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Not an easy question to answer. That stuff is mostly private and they seem to use greatly modified versions of some public plugins.
wordpresscom tag in repository has several.
Also see this topic for more ideas and places to check List of plugins used on WordPress.com