WriteFreely

 WriteFreely  is an open-source blogging platform that supports ActivityPub federation and plans to support Micropub in its desktop app and probably web editor too.

The creators' instance (Matt Baer) is at https://write.as/, open instances as well as fully managed hosted instances are available too. A list of the federated nodes, instances, users, and posts is available at https://the-federation.info/writefreely.

Written in Go, the source is available on GitHub.

Micropub plans

 * https://writing.exchange/@matt/101060189021992761
 * "@Gargron I'm planning on implementing it [Micropub] in WriteFreely, personally. It'd be awesome if Mastodon supported it too!" @matt November 12, 2018
 * https://writing.exchange/@matt/101066523112656548
 * "@eddiehinkle.com I would definitely like to support it working as a client -- probably in the web editor, but definitely in our native desktop apps." @matt November 13, 2018