Do you have a link to Comic List?

http://www.comiclist.com/

Well, the "underscore" is in fact a "td"... this one
table.tabs td.last { background:url(img/tabs_right_shadow.png) no-repeat right top; width: 4px; padding: 0; border-bottom:1px solid #9db0bc}
and it may well be effecting the widths and margins of the table and divs in the sidebar.

Check your HTML for an "empty/unused" td

I'm looking at the source for comiclist.com/index.php, and I can't find that "td." Where are you seeing it?

1) there is a dash or underscore on the top right of the page. Where in the world did this come from?

This is a background image for tabs. You can delete the following code from _body_header.inc.php since you don't use tabs

<div class="PageTop">
  <table class="tabs" cellspacing="0">
    <tr>
      <td class="first"></td>

      <?php
  // ------------------------------- CUSTOM TABS INCLUDED HERE -----------------------------
      skin_include( '_custom_tabs.inc.php' );
  // ----------------------------------- END OF CUSTOM TABS --------------------------------
      
// ----------------------------------- "Page Top" CONTAINER EMBEDDED HERE --------------------------
      skin_container( NT_('Page Top'), array(
              'block_start'         => '',
              'block_end'           => '',
              'block_display_title' => false,
              'list_start'          => '',
              'list_end'            => '',
              'item_start'          => '<td class="option">',
              'item_end'            => '</td>',
              'item_selected_start' => '<td class="current">',
              'item_selected_end'	  => '</td>',
          ) );
// --------------------------------------- END OF "Page Top" CONTAINER -----------------------------
      ?>

      <td class="last"></td>
    </tr>
  </table>
</div>

2) the right side is indented more than the left side. Why?

You have some [url=http://validator.w3.org/check?verbose=1&uri=http%3A%2F%2Fwww.comiclist.com%2F]validation errors[/url], and you should fix at least the following two:
1. Put this code in Free HTML widget

<form style="border:0px solid #ccc;padding:0px;text-align:center;" action="http://feedburner.google.com/fb/a/mailverify" method="post" target="popupwindow" onsubmit="window.open('http://feedburner.google.com/fb/a/mailverify?uri=comiclistfeed', 'popupwindow', 'scrollbars=yes,width=550,height=520');return true"><p>Enter your email address:</p><p><input type="text" style="width:140px" name="email"/></p><input type="hidden" value="comiclistfeed" name="uri"/><input type="hidden" name="loc" value="en_US"/><input type="submit" value="Subscribe" /><p>Delivered by <a href="http://feedburner.google.com" target="_blank">FeedBurner</a></p></form>

2. You should properly close commented blocks, replace this

<noscript>Member of the<a href="http://www.boxxet.com/" id="boxxetId_u12099">Boxxet Network</a></noscript>
<!-- End Boxxet Code -->
</center>
<br />
<!--
<a href="http://blogshares.com/blogs.php?blog=http%3A%2F%2Fwww.comiclist.com%2F&amp;user=50526"><img src="http://blogshares.com/images/blogshares.jpg" alt="Listed on BlogShares" width="117" height="23"/></a><script src="http://widgets.technorati.com/t.js" type="text/javascript"> </script>
<br /><br />
<a href="http://technorati.com/blogs/http://www.comiclist.com/?sub=tr_authority_t_ns" class="tr_authority_t_js" style="color:#4261DF">View blog authority</a>


</div>

<div class="bSideItem">

with this

<noscript>Member of the<a href="http://www.boxxet.com/" id="boxxetId_u12099">Boxxet Network</a></noscript>
<!-- End Boxxet Code -->
</center>
<br />
<!--
<a href="http://blogshares.com/blogs.php?blog=http%3A%2F%2Fwww.comiclist.com%2F&amp;user=50526"><img src="http://blogshares.com/images/blogshares.jpg" alt="Listed on BlogShares" width="117" height="23"/></a><script src="http://widgets.technorati.com/t.js" type="text/javascript"> </script>
<br /><br />
<a href="http://technorati.com/blogs/http://www.comiclist.com/?sub=tr_authority_t_ns" class="tr_authority_t_js" style="color:#4261DF">View blog authority</a>

-->
</div>

<!--
<div class="bSideItem">

ps please add a link to your site on your profile page

Thanks! I really just need to get rid of the blogshares, technorati, and boxxet links, as they don't serve a useful purpose anymore.

The site already looks much better, and I will work on the validation errors. Thanks everyone!