Plugin Tag: add buttons:

AddQuicktag

(44 total ratings)

This plugin makes it easy to add Quicktags to the html – and visual-editor.

Frank Bueltge 100 000+ active installations Tested with 5.7.12
Post Editor Buttons Fork

(11 total ratings)

This plugin allows you add your own buttons to the post editor's TEXT mode toolbar.

Kailey (trepmal) 1 000+ active installations Tested with 4.6.29
Post Editor Buttons

(0 total ratings)

This plugin allows you add your own buttons to the post editor's toolbar.

Oren Yomtov 100+ active installations Tested with 2.9.2
XHE Quicktags

(0 total ratings)

This plugin makes it easy to add Quicktags to the html – and visual-editor.

xhtmlexpert Fewer than 10 active installations Tested with 5.7.12