<div id="username" style="display:none;">$block[username]</div>
<script type="text/javascript">
$(document).ready(function(){
 var prefix = "Hey ";
 var suffix = ", welcome to our rockin' website!"
 
// don't change anything below
 var newtitle = prefix + $('#username').text() + suffix;
 $('#username').closest('div.ContentBox').find('.ContentBoxTitle,.ContentBoxTitleNoBackground').text(newtitle);
})
</script>

• Note that this script replaces the entire box title. In the screen shot above, the box title was "Welcome". The script changed it to "Hey {username}, welcome to our website!"
• The variable named prefix will be added before the user name
• The variable named suffix added the text after the user name, there really isn't that much to it.
• Note: You can add HTML to the prefix and suffix variables (like <img>) if you wish.

Before
After

<div id="username" style="display:none;">$block[username]</div>
<script type="text/javascript">
$(function(){
    // replaces "Guest", if someone isn't logged in
    var defaultName = "friend";

    // Don't change anything below
    // ***************************
    var user = $('#username').text();
    user = (user === "Guest") ? defaultName : user;
    $('.ContentBoxTitle,.ContentBoxTitleNoBackground').each(function(){
        $(this).text(function(i,t){
            return t.replace(/\[user\]/g, user);
        });
    });
});
</script>

• Change any widget titles by editing the widget, then clicking on the underlined "Widget Title" in the upper right corner.
• Add "[user]" where ever you want the user name to appear
• Click the [ Update Title ] button
• Add the above code to your header or footer area (Control Panel > Custom HTML & Script > Footer Area) - I would say it is preferred to put this script in the header area (subscribed guilds) because the script runs sooner. Otherwise the [user] doesn't get changed until the page has loaded the content - it may be noticeable.
• Modify the defaultName (in blue) with whatever default name you want guests (people not logged in) to see - be nice!

<div id="username" style="display:none;">$block[username]</div>
<script type="text/javascript">
$(function(){
    // replaces "Guest", if someone isn't logged in
    var defaultName = "friend";

    // Don't change anything below
    // ***************************
    var user = $('#username').text();
    user = (user === "Guest") ? defaultName : user;
    $('.ContentBoxTitle,.ContentBoxTitleNoBackground,.ContentBoxBody').filter(':contains("\[user\]")').each(function(){
      // abort if the element contains a script tag
      if (!$(this).find('script').length) {
        $(this).html(function(i,t){
          return t.replace(/\[user\]/g, user);
        });
      }
    });

});
</script>