Jesper Johansen (jayjdk)

This plugin, Jayj Quicktag, allows you easily to add custom Quicktags to the post, page, or a custom editor.
It adds a settings page where you can add all the Quicktags you want (see Screenshots)
It supports both import and export of your Quicktags so you easily can add them to another WordPress install.

Jayj Quicktag

Allows you easily to add custom Quicktags to the post editor.

xhtmlexpert

Allows you to add buttons to the WordPress admin text editor.

XHE Quicktags

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

Jayj Quicktag vs XHE Quicktags