Send email to website owner after user registration I have a client that wants to get notified whenever someone registers on their website. 

How can I do this, is there some kind of plugin? If not how would I go about doing this?
I have done something very similar. You'll want to create a plugin for this. 

In my case, I am validating email addresses so I was checking against the onBeforeActivateUser event. If you are not validating email addresses, @Lindsey D' answer is where you want to go.

Either way, they will look about like this:
I created a plugin called accounts.

MyPlugin.php

public function init()
{
    /**
     * Event listener for activating a user.
     *
     * @param Event $event
     */
    craft()-&gt;on('users.onBeforeActivateUser', function (Event $event) {
        $user = $event-&gt;params['user'];

        craft()-&gt;accounts-&gt;onSendNotificationEmail($user);
    });
}


MyService.php

/**
 * Sends a notification email.
 *
 * @param $user
 * @return bool
 */
public function onSendNotificationEmail($user)
{
    $recipient = $user-&gt;email; // get the email address passed in

    $email = new EmailModel();

    $message = '&lt;!DOCTYPE html&gt;&lt;html&gt;&lt;title&gt;My Title&lt;/title&gt;&lt;head&gt;';
    $message .= '&lt;body&gt;';
    $message .= '&lt;table width="800" border="0" cellpadding="0" cellspacing="0"&gt;';
    $message .= '&lt;tr&gt;&lt;td style="text-align: center;"&gt;LOGO/BRANDING &lt;/td&gt;&lt;/tr&gt;';
    $message .= '&lt;/table&gt;';
    $message .= '&lt;p&gt;Hello,&lt;br&gt;There has been a new account created on the website.&lt;/p&gt;';
    $message .= '&lt;p&gt;...your message here...&lt;/p&gt;';
    $message .= '&lt;/body&gt;&lt;/html&gt;';

    $email-&gt;subject = 'New Account Created';
    $email-&gt;body = $message;

    try {
        $email-&gt;toEmail = $recipient;

        craft()-&gt;email-&gt;sendEmail($email);
    } catch (\Exception $e) {
        return false;
    }
}


I'm sure there is a cleaner way to get your email body into the CP itself and allow the admin to change the message easily, but this works quick/well.