Adding TinyMCE style selector, that plays nice with others. Checks if a style selector has already been added to row 2, and also allows for styles that might already exist from other code / theme / plugin. Also internationalizes displayed labels.

gistfile1.aw

<?php

add_filter( 'mce_buttons_2', 'wps_mce_buttons_2' );
/**
 * Show the style dropdown on the second row of the editor toolbar.
 *
 * This code also adds the font family and font size dropdowns too, along with a horizontal rule button.
 *
 * @param  array $buttons Exising buttons
 * @return array          Amended buttons 
 */
function wps_mce_buttons_2( array $buttons ) {
	
	$additional_buttons = array( 'styleselect', 'fontselect', 'fontsizeselect', 'hr' );
	
	return array_unique( array_merge( $buttons, $additional_buttons ) );

}

add_filter( 'tiny_mce_before_init', 'wps_mce_before_init' );
/**
 * Add column entries to the style dropdown.
 * 
 * 'text-domain' should be replaced with your theme or plugin text domain for
 * translations.
 *
 * @param  array $settings Existing settings for all toolbar items
 * @return array           Amended settings
 */
function wps_mce_before_init( array $settings ) {

	$style_formats = array(
		
		array( 'title' => __( 'Columns', 'text-domain' ), ),
		array(
			'title'   => __( 'First Half', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-half first',
		),
		array(
			'title'   => __( 'Half', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-half',
		),
		array(
			'title'   => __( 'First Third', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-third first',
		),
		array(
			'title'   => __( 'Third', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-third',
		),
		array(
			'title'   => __( 'First Quarter', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-fourth first',
		),
		array(
			'title'   => _( 'Quarter', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-fourth',
		),
		array(
			'title'   => __( 'First Fifth', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-fifth first',
		),
		array(
			'title'   => __( 'Fifth', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-fifth',
		),
		array(
			'title'   => __( 'First Sixth', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-sixth first',
		),
		array(
			'title'   => __( 'Sixth', 'text-domain' ),
			'block'   => 'div',
			'classes' => 'one-sixth',
		),
	);

	// Check if there are some styles already
	if ( isset( $settings['style_formats'] ) ) {		
		// Decode any existing style formats
		$existing_style_formats = json_decode( $settings['style_formats'] );

		// Merge our new formats with any existing formats and re-encode
		$settings['style_formats'] = json_encode( array_merge( (array) $existing_style_formats, $style_formats ) );
	} else {
		$settings['style_formats'] = json_encode( $style_formats );
	}

	return $settings;

}