Identifier: quickreply_editor_message_box_before
Prosilver Location (if applicable): quickreply_editor.html, right above the line

<div id="message-box">

Subsilver Location (if applicable): no need.
Explanation: this event allows extensions to modify quick reply postform. For example, add BBCode buttons and/or smilies and so on.
Use case: we have at least 1 extension in development that modifies quick reply postform in the way above.