Is the editor used by Stackoverflow for textbox a Jquery plugin or is it a custom designed editor just for the site? Are there any alternatives to this editor. I mean a editor that has programming syntax friendly features and simple for users to use.
Have a look into SyntaxHL plugin for Tinymce
From the mouth of developer:-
SyntaxHL is a plugin for the Tinymce (http://tinymce.moxiecode.com/) javascript WYSIWYG editor allowing you to insert highlighted code into the editor. The syntax highlighting is done using Syntax Highligher (http://code.google.com/p/syntaxhighlighter/) and the plugin simply produces the right syntax for this.
Lastly, what I know is a plugin syntaxhighlighter-plugin-for-tinymce-wysiwyg-editor.aspx from Nawaf's Blog, for which i don't have much idea(you can have a look though)!
EDIT:-
Was just googling around and found one question in meta.stackoverflow, which is what-wysiwyg-editor-is-stack-overflow-using, over here someone posted a link introducing-griffin-editor-a-jquery-textarea-plugin which was quiet good to read and implement.
Just have a look if it helps!
Edit 2:-
While reading, I found people mentioning WMD Editor as the text-editor for stackoverflow. Which is not longer available in wmd-editor.com.