Jon Cave

If you don’t check your admin panel on your WordPress install very often (maybe because you prefer to use remote publishing) or you want to make sure 
that your clients’ WordPress installations are updated, then this is the plugin for you. You don’t have to login to your admin panel regularly, 
suscribe to an RSS feed, or do anything apart from installing this plugin to be notified when an update to WordPress is released.
All you have to do is install Update Notifier and forget it until you receive an email telling you to update.
To change Update Notifier’s options, go to Update Notifier under the main Settings menu. From there you can add a secondary email address 
which will also receive update notifications and you can activate update notifications for themes and plugins.

Update Notifier

Sends email notifications if a new version of WordPress available. Notifications about updates for plugins and themes can also be sent.

Alley

Scott Cariss

Monitors your WordPress installation for core, plugin and theme updates and emails you when they are available. This plugin is ideal if you don’t login to your WordPress admin regularly or you support a client’s website.
Features
<ul>
<li>Set the interval of how often to check for updates; hourly, twice daily or daily.</li>
<li>Sets WordPress to check for updates more often meaning you get to know about updates sooner.</li>
<li>Get emailed about core, plugin and theme updates.</li>
<li>Chose if you want to be notified about active only themes and plugins updates.</li>
<li>Remove upgrade nag message to non-admin users.</li>
<li>For advanced users there are a number of filters and actions you can use. More coming soon.</li>
</ul>
This plugin is a fork of <a href="https://wordpress.org/extend/plugins/update-notifier/" rel="ugc">Update Notifier</a>. This plugin was forked because there seemed to be no further development on the existing plugin and there was no way to contact the original author to ask about taking ownership. WP Updates Notifier has the following improvements over Updates Notifier:
<ul>
<li>Completely rewritten from the ground up using best practises for writing WordPress plugins</li>
<li>Code wrapped in a class so better namespace.</li>
<li>You can set the cron interval, allowing for more frequent checks.</li>
<li>Update checks trigger WordPress internal update check before notification.</li>
<li>Allows you to set the ‘from address’.</li>
<li>Allows you to set multiple ‘to addresses’.</li>
<li>Makes use of the Settings API.</li>
<li>A number of available hooks and filters for advanced users.</li>
<li>Active support and development.</li>
</ul>
Languages
<ul>
<li>French by <a href="http://www.ordilibre.com/" rel="nofollow ugc">Christophe Catarina</a> – Added 03 July 2013</li>
<li>German by <a href="http://alexander.pfabel.de/" rel="nofollow ugc">Alexander Pfabel</a> – Added 02 October 2012</li>
</ul>
<h3>Filters</h3>
Two filters have been provided to allow you to alter the email subject and email content being sent by WP Updates Notifier.
<h4>sc_wpun_email_subject</h4>
@parameters: 
$email_subject – the email subject to be filtered.
Example:
<pre><code>/* 
* Alter the email subject being sent by WP Updates Notifier 
*/
function alter_wp_updates_notifier_email_subject( $email_subject ) {
 $email_subject = 'This is the new email subject for updates notifier';
 return $email_subject;
}
add_filter( 'sc_wpun_email_subject', 'alter_wp_updates_notifier_email_subject' );
</code></pre>
<h4>sc_wpun_email_content</h4>
@parameters: 
$message – the content of the email to be filtered
Example:
<pre><code>/* 
* Alter the email content being sent by WP Updates Notifier 
*/
function alter_wp_updates_notifier_email_content( $message ) {
 $message = 'This is our new email content that will override the default.';
 return $message;
}
add_filter( 'sc_wpun_email_content', 'alter_wp_updates_notifier_email_content' );
</code></pre>

WP Updates Notifier

It's ok for theme and plugins notifications, but notification about core updates does't work for more than one year!
This plugin probably won't recieve an update anymore.

Out-of date and not fully working

Sends email to notify you if there are any updates for your WordPress site. Can notify about core, plugin and theme updates.

Update Notifier vs WP Updates Notifier