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.

Oren Yomtov

1.7: 
WordPress 2.9.1 compatible.

This plugins allows you to add your own buttons to the post edtor’s toolbar. 
I’m talking about the buttons that you see above your content text area when editing posts (b,i,link,img,ul,li). 
For example you can add a “h3” button to make subheaders in your post. 
There are virtually endless buttons you can add.

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Post Editor Buttons

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

XHE Quicktags vs Post Editor Buttons