Allan Bruford

Get notified when any of the following events occur:
<ul>
<li>A PHP error or warning occurs (monitors E_ERROR | E_PARSE | E_CORE_ERROR [see http://php.net/manual/en/errorfunc.constants.php for more information])</li>
<li>Plugin is either activated or deactivated (along with the user who performed the action)</li>
</ul>
Created by <a href="http://allanbruford.com/" rel="nofollow ugc">Allan Bruford</a> – Software Engineer at <a href="http://quickenloans.com" rel="nofollow ugc">Quicken Loans</a>, Detroit, MI

WP Activity Logger

WP Activity Logger is a plugin that monitors and notifies of certain events that take place on your site.

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.

WP Activity Logger vs Update Notifier