Thanks to the work done by Andrew Ozz et al., adding a front-end editor in WordPress 3.3 is very simple.

Syntax:

wp_editor( $content, $editor_id, $settings = array() );

// default settings
$settings =   array(
	'wpautop' => true, // use wpautop?
	'media_buttons' => true, // show insert/upload button(s)
	'textarea_name' => $editor_id, // set the textarea name to something different, square brackets [] can be used here
	'textarea_rows' => get_option('default_post_edit_rows', 10), // rows="..."
	'tabindex' => '',
	'editor_css' => '', // intended for extra styles for both visual and HTML editors buttons, needs to include the <style> tags, can use "scoped".
	'editor_class' => '', // add extra class(es) to the editor textarea
	'teeny' => false, // output the minimal editor config used in Press This
	'dfw' => false, // replace the default fullscreen with DFW (needs specific css)
	'tinymce' => true, // load TinyMCE, can be used to pass settings directly to TinyMCE using an array()
	'quicktags' => true // load Quicktags, can be used to pass settings directly to Quicktags using an array()
);

View the wp_editor() examples