Demonstration plugin for my Introduction to WordPress Plugin Development course for Pluralsight. Download the full source here: http://cl.ly/3u153a2N1v32

ps-demo-plugin.php

<?php
/*
Plugin Name: Pluralsight plugin development demo
Description: This is a demonstration plugin for the Introduction to WordPress Plugin Development course on Pluralsight.
Author: Chris Reynolds
Version: 1.0
*/

function ps_plugindev_options_page() {
  ?>
		<div class="wrap">
			<?php screen_icon(); ?>
			<h2><?php _e( 'Pluralsight Plugin Development Demo', 'ps-demo' ); ?></h2>
			<p><?php _e( 'This is a demonstration plugin for Introduction to WordPress Plugin Development. This plugin will add a sharing link underneath the post content.', 'ps-demo' ); ?></p>
			<form method="POST" action="options.php">
				<?php
					settings_fields( 'ps_plugindev_settings' );
				?>
				<table class="form-table">
					<tbody>
						<?php ps_plugindev_do_options(); ?>
					</tbody>
				</table>
				<input type="submit" class="button-primary" value="<?php _e( 'Save Options', 'ps-demo' ); ?>" />
				<input type="hidden" name="ps-plugindev-submit" value="Y" />
			</form>
		</div>
	<?php
}

function ps_plugindev_menu() {
	add_submenu_page( 'options-general.php', __( 'Plugin Demo', 'ps-demo' ), __( 'Plugin Demo', 'ps-demo' ), 'administrator', 'ps_plugindev', 'ps_plugindev_options_page' );
}
add_action( 'admin_menu', 'ps_plugindev_menu' );

function ps_plugindev_init() {
	register_setting( 'ps_plugindev_settings', 'ps_plugindev', 'ps_plugindev_validate' );
}
add_action( 'admin_init', 'ps_plugindev_init' );

function ps_plugindev_do_options() {
	$options = get_option( 'ps_plugindev' );
	ob_start();
	?>
		<tr valign="top"><th scope="row"><?php _e( 'Sharing text', 'ps-demo' ); ?></th>
			<td>
				<?php
					if ( $options['sharing-text'] ) {
						$options['sharing-text'] = wp_kses($options['sharing-text']);
					} else {
						$options['sharing-text'] = __( 'Share this post', 'ps-demo' );
					}
				?>
				<input type="text" id="sharing-text" name="ps_plugindev[sharing-text]" width="30" value="<?php echo $options['sharing-text']; ?>" /><br />
				<label class="description" for="ps_plugindev[sharing-text]"><?php _e( 'Allows you to change the text that displays next to the sharing buttons', 'ps-demo' ); ?></label>
			</td>
		</tr>
		<tr valign="top"><th scope="row"><?php _e( 'What services do you want to share to?', 'ps-demo' ); ?></th>
			<td>
				<?php
					foreach ( ps_plugindev_services() as $service ) {
						$label = $service['label'];
						$value = $service['value'];
						echo '<label><input type="checkbox" name="ps_plugindev[' . $value . '] value="1" ';
						switch ($value) {
							case 'google' :
								if ( isset( $options['google'] ) ) { checked( 'on', $options['google'] ); }
								break;
							case 'twitter' :
								if ( isset( $options['twitter'] ) ) { checked( 'on', $options['twitter'] ); }
								break;
							case 'facebook' :
								if ( isset( $options['facebook'] ) ) { checked( 'on', $options['facebook'] ); }
								break;
							case 'stumbleupon' :
								if ( isset( $options['stumbleupon'] ) ) { checked( 'on', $options['stumbleupon'] ); }
								break;
							case 'digg' :
								if ( isset( $options['digg'] ) ) { checked( 'on', $options['digg'] ); }
								break;
							case 'email' :
								if ( isset( $options['email'] ) ) { checked( 'on', $options['email'] ); }
								break;
						}
						echo '/> ' . esc_attr($label) . '</label><br />';
					}
				?>
			</td>
	<?php
}

function ps_plugindev_services() {
	$services = array(
		'google' => array(
			'value' => 'google',
			'label' => __( 'Google+', 'ps-demo' )
		),
		'twitter' => array(
			'value' => 'twitter',
			'label' => __( 'Twitter', 'ps-demo' )
		),
		'facebook' => array(
			'value' => 'facebook',
			'label' => __( 'Facebook', 'ps-demo' )
		),
		'stumbleupon' => array(
			'value' => 'stumbleupon',
			'label' => __( 'StumbleUpon', 'ps-demo' )
		),
		'digg' => array(
			'value' => 'digg',
			'label' => __( 'Digg', 'ps-demo' )
		),
		'email' => array(
			'value' => 'email',
			'label' => __( 'Email', 'ps-demo' )
		),
	);
	return $services;
}

function ps_plugindev_content_filter( $content ) {
	$options = get_option( 'ps_plugindev' );

	if ( !$options['sharing-text'] ) {
		$options['sharing-text'] = __( 'Share this post', 'ps-demo' );
	}

	$new_content = '<div class="ps-plugindev-sharing postmeta"><strong>' . $options['sharing-text'] . '</strong><br />';

	if ( isset( $options['google'] ) ) {
		$new_content .= ps_plugindev_do_google_button();
	}
	if ( isset( $options['twitter'] ) ) {
		$new_content .= ps_plugindev_do_twitter_button();
	}
	if ( isset( $options['facebook'] ) ) {
		$new_content .= ps_plugindev_do_facebook_button();
	}
	if ( isset( $options['stumbleupon'] ) ) {
		$new_content .= ps_plugindev_do_stumbleupon_button();
	}
	if ( isset( $options['digg'] ) ) {
		$new_content .= ps_plugindev_do_digg_button();
	}
	if ( isset( $options['email'] ) ) {
		$new_content .= ps_plugindev_do_email_button();
	}
	$new_content .= '</div>';

	return $content . $new_content;
}
add_filter( 'the_content', 'ps_plugindev_content_filter' );

function ps_plugindev_do_google_button() {
	$button = '<div id="google"><g:plusone size="medium"></g:plusone></div>';
	return $button;
}

function ps_plugindev_do_twitter_button() {
	$button = '<div id="twitter"><a href="https://twitter.com/share" class="twitter-share-button">Tweet</a></div>';
	$button .= '<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0],p=/^http:/.test(d.location)?\'http\':\'https\';if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src=p+\'://platform.twitter.com/widgets.js\';fjs.parentNode.insertBefore(js,fjs);}}(document, \'script\', \'twitter-wjs\');</script>';

	return $button;
}

function ps_plugindev_do_facebook_button() {
	global $post;

	$button = '<div id="facebook" class="fb-like" data-href="' . get_permalink( $post->ID ) . '" data-send="false" data-layout="button_count" data-width="100" data-show-faces="false"></div>';

	return $button;
}

function ps_plugindev_do_stumbleupon_button() {
	$button = '<div id="stumbleupon"><su:badge layout="1"></su:badge></div>';

	return $button;
}

function ps_plugindev_do_digg_button() {
	global $post;
	$link = urlencode( get_permalink( $post->ID ) );
	$image = plugins_url( 'img/100x20-digg-button.gif', __FILE__ );

	$button = '<div id="digg"><a href="http://digg.com/submit?phase=2&url=' . $link . '"><img src="' . $image . '" height="16" /></a></div>';

	return $button;
}

function ps_plugindev_do_email_button() {
	global $post;
	$title = get_the_title( $post->ID );
	$image = plugins_url( 'img/email-icon.gif', __FILE__ );

	$button = '<div id="email"><a href="mailto:?subject=' . $title . '&body=This is a cool post: &lt;a href=' . get_permalink( $post->ID ) . '&gt;' . $title . '&lt;/a&gt;"><img src="' . $image . '" /> ' . __( 'Email this', 'ps-demo' ) . '</a></div>';

	return $button;
}

function ps_plugindev_head_scripts() {
	$options = get_option( 'ps_plugindev' );
	if ( isset( $options['facebook'] ) && !is_admin() ) {
		?>
		<div id="fb-root"></div>
		<script>(function(d, s, id) {
			var js, fjs = d.getElementsByTagName(s)[0];
			if (d.getElementById(id)) return;
			js = d.createElement(s); js.id = id;
			js.src = "//connect.facebook.net/en_US/all.js#xfbml=1&appId=448961491860552";
			fjs.parentNode.insertBefore(js, fjs);
		}(document, 'script', 'facebook-jssdk'));</script>
		<?php
	}
	if ( isset( $options['stumbleupon'] ) && !is_admin() ) {
		?>
		<script type="text/javascript">
			(function() {
				var li = document.createElement('script'); li.type = 'text/javascript'; li.async = true;
				li.src = ('https:' == document.location.protocol ? 'https:' : 'http:') + '//platform.stumbleupon.com/1/widgets.js';
				var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(li, s);
			})();
		</script>
		<?php
	}
}
add_action( 'wp_head', 'ps_plugindev_head_scripts' );

function ps_plugindev_scripts() {
	$options = get_option( 'ps_plugindev' );

	if ( !is_admin() ) {
		if ( isset( $options['google'] ) ) {
			wp_enqueue_script( 'googleplus', 'https://apis.google.com/js/plusone.js' );
		}
		wp_enqueue_style( 'ps-plugindev', plugins_url( 'css/style.css', __FILE__ ) );
	}
}
add_action( 'wp_print_scripts', 'ps_plugindev_scripts' );

function ps_plugindev_validate($input) {
	$input['sharing-text'] = wp_filter_nohtml_kses( $input['sharing-text'] );
	foreach ( ps_plugindev_services() as $service ) {
		$value = $service['value'];
		switch ($value) {
			case 'google' :
				if ( !array_key_exists( $input['google'], $value ) )
					$input['google'] = $input['google'];
				break;
			case 'twitter' :
				if ( !array_key_exists( $input['twitter'], $value ) )
					$input['twitter'] = $input['twitter'];
				break;
			case 'facebook' :
				if ( !array_key_exists( $input['facebook'], $value ) )
					$input['facebook'] = $input['facebook'];
				break;
			case 'stumbleupon' :
				if ( !array_key_exists( $input['stumbleupon'], $value ) )
					$input['stumbleupon'] = $input['stumbleupon'];
				break;
			case 'digg' :
				if ( !array_key_exists( $input['digg'], $value ) )
					$input['digg'] = $input['digg'];
				break;
			case 'email' :
				if ( !array_key_exists( $input['email'], $value ) )
					$input['email'] = $input['email'];
				break;
		}
	}

	return $input;
}

style.css

/* some styles for the sharing buttons */
.ps-plugindev-sharing #facebook,
.ps-plugindev-sharing #stumbleupon,
.ps-plugindev-sharing #digg,
.ps-plugindev-sharing #email{
  float: left;
	margin-right: 20px;
}
.ps-plugindev-sharing #google,
.ps-plugindev-sharing #twitter {
	float: left;
}
.ps-plugindev-sharing {
	clear: both;
}

WordPress update :Missing argument 2 for wp_kses()
Old
$options['sharing-text'] = wp_kses($options['sharing-text']);
New
$options['sharing-text'] = wp_kses($options['sharing-text'], array());