Ever wanted to add custom headers and footers in the Content Query Web Part? Here is a quick sample of how to add both. Thanks to Zac Orndorff for figuring this out. 

Use this in an itemstyle to output a header:

<xsl:if test="count(preceding-sibling::*)=0">
</xsl:if>

Use this in an itemstyle to output a footer:

<xsl:if test="count(following-sibling::*)=0">
</xsl:if>

A Sample Item Style

  <xsl:template name="HeaderFooter" match="Row[@Style='HeaderFooter']" mode="itemstyle"><xsl:variable name="SafeLinkUrl">
            <xsl:call-template name="OuterTemplate.GetSafeLink">
                <xsl:with-param name="UrlColumnName" select="'LinkUrl'"/>
            </xsl:call-template>
        </xsl:variable>
        <xsl:variable name="SafeImageUrl">
            <xsl:call-template name="OuterTemplate.GetSafeStaticUrl">
                <xsl:with-param name="UrlColumnName" select="'ImageUrl'"/>
            </xsl:call-template>
        </xsl:variable>
        <xsl:variable name="DisplayTitle">
            <xsl:call-template name="OuterTemplate.GetTitle">
                <xsl:with-param name="Title" select="@Title"/>
                <xsl:with-param name="UrlColumnName" select="'LinkUrl'"/>
            </xsl:call-template>
        </xsl:variable>
        <!-- header -->
        <xsl:if test="count(preceding-sibling::*)=0">
<div class="header">
<div class="header-row">
<div class="header-column title">Title</div>
<div class="header-column description">Description</div>
</div>
</div>
</xsl:if>
        <!-- end header -->
        <div class="item">
           <div class="link-item">
              <xsl:call-template name="OuterTemplate.CallPresenceStatusIconTemplate"/>
                <a href="{$SafeLinkUrl}" title="{@LinkToolTip}">
                  <xsl:if test="$ItemsHaveStreams = 'True'">
                    <xsl:attribute name="onclick">
                      <xsl:value-of select="@OnClickForWebRendering"/>
                    </xsl:attribute>
                  </xsl:if>
                  <xsl:if test="$ItemsHaveStreams != 'True' and @OpenInNewWindow = 'True'">
                    <xsl:attribute name="onclick">
                      <xsl:value-of disable-output-escaping="yes" select="$OnClickTargetAttribute"/>
                    </xsl:attribute>
                  </xsl:if>
                  <xsl:value-of select="$DisplayTitle"/>
                </a>
                <div class="description">
                    <xsl:value-of select="@Description" />
                </div>
            </div>
        </div>
        <!-- footer -->
<xsl:if test="count(following-sibling::*)=0">
<div class="footer-row">
<div class="footer-column view-all">
<a href="#">View All Items</a>
</div>
</div>
</xsl:if>      
<!-- end footer -->  
    </xsl:template>

Some CSS to clean up the display:

.header-row, .footer-row {

background: url("/_layouts/images/selbg.png") repeat-x left top;
background-color: #F6F6F6;
vertical-align: middle;
min-height: 25px;
border-top: 1px solid #E0E0E0;
border-bottom: 1px solid #B8BABD;
border-left: 1px solid #E0E0E0;
border-right: 1px solid #B8BABD;
}
.header-column, .footer-column {
display: inline-block;
padding: 0.5em 2.2em 0.5em 0.5em;
min-width:200px;
}
.item {
border-left: 1px solid #E0E0E0;
border-right: 1px solid #B8BABD;
}
.footer-column.view-all {
text-align:center;
width:100%;
}