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.

kkarpieszuk

This plugin is for you if you don’t look inside of your Admin Panel every day (for example you have tens of wordpress installations) but still want to have wordpress up to date. After installation plugin will check every day if newer version of wordpress is available and if yes, will send email to blog’s admin with notification.

Upgrade Notification by Email

Sends daily notofication at admins' email if installation of Wordpress is out of date

Update Notifier vs Upgrade Notification by Email