Create a "Fork me on GitHub" ribbon.

forkmeribbon_wp_shortcode.php

<?php
/**
 * Create a "Fork me on GitHub" ribbon.
 *
 * @example [forkmeribbon url="http://www.example.com" color="red" position="right"]
 * @since 1.0
 */
function forkmeribbon_function( $atts ) {
	extract( shortcode_atts( array(
		'url'      => '',
		'color'    => 'red',
		'position' => 'right'
	), $atts ) );

	/*
     * Convert the value of $color into the value used by GitHub.
	 * The possible values are defined the $colors array below.
     */
	$colors = array(
		'red'      => 'red_aa0000',
		'green'    => 'green_007200',
		'darkblue' => 'darkblue_121621',
		'orange'   => 'orange_ff7600',
		'gray'     => 'gray_6d6d6d',
		'white'    => 'white_ffffff'
	);
	if ( array_key_exists( $color, $colors ) ) {
		// If $color exists in $colors array, get the corresponding value...
		$color_output = $colors[$color];
	} else {
		// ... otherwise assign a default value.
		$color_output = 'red_aa0000';
	}

	// Check if $position has an allowed value.
	$positions = array( 'left', 'right' );
	if( ! in_array( $position, $positions ) ) {
		$position = 'right';
	}

    // Build the output.
	$output = '<a href="' . $url . '"><img style="position: absolute; top: 0; ' . $position . ': 0; border: 0; border-radius: 0; box-shadow: none; z-index: 1;" src="https://s3.amazonaws.com/github/ribbons/forkme_' . $position . '_' . $color_output . '.png" alt="Fork me on GitHub"></a>';

	return $output;
}
add_shortcode( 'forkmeribbon', 'forkmeribbon_function' );