WP MarkItUp is not bad at all… Runs on the standard MarkItUp code, driven by jQuery and is a WP plugin.
Tried it out by bodging in a Paypal “buy now” button to the editor, and it does ok, and would make putting in various extra functions of this sort a reasonably modular experience for the average programmer.
If you do know what you’re doing with javascript/AJAX, not sure it’s a necessary replacement, but it might provide a conduit for cleaning up the editor/quicktags functions, never-ending…
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Wordpress Plugin - WP MarkItUp