I didn't realize I hadn't mentioned this plugin. It's more for plugin authors than for general WordPress users, but they may yet be interested.
This plugin shows you how to easily add new buttons to the Quicktags toolbar without modifying quicktags.js, just by dropping in and activating a plugin.
You could use it to provide an interface for a real spellchecker, or to insert a tag that's not already on the bar, or to provide a button to open an inmage browser, or any number of editing-related things.
It's called Edit Button Temlpate. Use it how you wish, if you wish.