WordPress
Updates to this list can now be added on the wiki, where it is being actively maintained: https://gitlab.com/fediverse/fediverse.gitlab.io/wikis/wordpress-integration-with-the-fediverse
WordPress is a mature free code project with a huge user base. Getting fediverse integration for comments working smoothly in WordPress would be a coup. There are a number of plug-ins that provide some pieces of the puzzle, but it might require a bit of community facilitation to herd all these cats into working on one plug-in (or suite of plug-ins?) that does a good job of everything.
WordPress plug-ins that may be relevant
Listed in order of most recent commits (at time of writing list 2018-05-08):
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Mastodon Auto Share - 2 weeks ago
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Autopost to Mastodon - 2 months ago
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ActivityStream extension - 3 months ago
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OStatus for Wordpress - 5 months ago
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GNU Social Autpost - 8 months ago
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Mastodon Embed - 1 year ago
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WP-GNU Social - 2 years ago
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GNU Social followers widget - 2 years ago