Send mail to users including receiver's username
There isn't a setting for that, but when a user sends an email to another user, using the contact form of the other user's account, the Contact module sends a message that start with (I sent an email to myself from Drupal.org):


  kiamlaluno,  
  
  kiamlaluno (http://drupal.org/user/55077) has sent you a message via your contact form (http://drupal.org/user/55077/contact) at drupal.org.  
  
  If you don't want to receive such e-mails, you can change your settings at http://drupal.org/user/55077.


The first line contain the receiver's username.

The following snippet is part of the code present in contact_mail(), the function that alter some of the messages sent from Drupal.

case 'user_mail':
case 'user_copy':
  $user = $params['user'];
  $account = $params['account'];
  $message['subject'] .= '[' . variable_get('site_name', 'Drupal') . '] ' . $params['subject'];
  $message['body'][] = "$account-&gt;name,";
  $message['body'][] = t("!name (!name-url) has sent you a message via your contact form (!form-url) at !site.", array('!name' =&gt; $user-&gt;name, '!name-url' =&gt; url("user/$user-&gt;uid", array('absolute' =&gt; TRUE, 'language' =&gt; $language)), '!form-url' =&gt; url($_GET['q'], array('absolute' =&gt; TRUE, 'language' =&gt; $language)), '!site' =&gt; variable_get('site_name', 'Drupal')), $language-&gt;language);
  $message['body'][] = t("If you don't want to receive such e-mails, you can change your settings at !url.", array('!url' =&gt; url("user/$account-&gt;uid", array('absolute' =&gt; TRUE, 'language' =&gt; $language))), $language-&gt;language);
  $message['body'][] = t('Message:', NULL, $language-&gt;language);
  $message['body'][] = $params['message'];
  break;


Other modules can change the content for the sent emails, if they implement hook_mail_alter(). This means that other modules could remove content from the email sent via the contact form; it also means that another module could replace the username used in the first line with the first and last name provided by the receivers in their user profile.