A lot of times you will want to display only unique values inside the Data View.  What I have here is a variation of the Muenchian Method using an <xsl:key>.  Check out jenitennison.com for a few variations on this.

First, browse out to your Announcements list (which you get by default when you create a SharePoint site), and add a few Announcements all with the same title (just for demonstrations sake :-))

Then...

1. Insert a Data View Web Part based on the Announcements list
2. Probably best to switch to code view at this point
3. Find the code that looks like this:
<xsl:template name="dvt_1.body">
  <xsl:param name="Rows"/>
  <xsl:param name="FirstRow"/>
  <xsl:param name="LastRow"/>
  <xsl:for-each select="$Rows">
   <xsl:variable name="KeepItemsTogether" select="false()"/>
   <xsl:variable name="HideGroupDetail" select="false()"/>
   <xsl:variable name="GroupStyle" select="'auto'"/>
   <xsl:if test="(position() &gt;= $FirstRow and position() &lt;= $LastRow) or $KeepItemsTogether">
    <xsl:if test="not($HideGroupDetail)" ddwrt:cf_ignore="1">
     <tr style="display:{$GroupStyle}">
      <td class="ms-vb">
       <xsl:value-of select="@Title"/>
      </td>
      <td class="ms-vb">
       <xsl:value-of select="@Editor"/>
      </td>
      <td class="ms-vb">
       <xsl:value-of select="ddwrt:FormatDate(string(@Modified), 1033, 5)"/>
      </td>
     </tr>
    </xsl:if>
   </xsl:if>
  </xsl:for-each>
 </xsl:template>

4. Change that code to look like this:
 <xsl:template name="dvt_1.body">
  <xsl:param name="Rows"/>
  <xsl:param name="FirstRow"/>
  <xsl:param name="LastRow"/>
  <xsl:for-each select="$Rows">
   <xsl:variable name="KeepItemsTogether" select="false()"/>
   <xsl:variable name="HideGroupDetail" select="false()"/>
   <xsl:variable name="GroupStyle" select="'auto'"/>
   <xsl:variable name="uniqueTitle" select="@Title"/> <!-- this is added to create a variable containing the Title -->
   <xsl:if test="(position() &gt;= $FirstRow and position() &lt;= $LastRow) or $KeepItemsTogether">
    <xsl:if test="not($HideGroupDetail)" ddwrt:cf_ignore="1">
     <tr style="display:{$GroupStyle}">
      <td class="ms-vb"><!-- this next part is where we compare the variable created above to the current selection and if they are different, we write the value -->
        <xsl:if test="generate-id()=generate-id(//Rows/Row[@Title=$uniqueTitle][1])">
           <xsl:value-of select="@Title"/>
        </xsl:if>
      </td>
      <td class="ms-vb">
       <xsl:value-of select="@Editor"/>
      </td>
      <td class="ms-vb">
       <xsl:value-of select="ddwrt:FormatDate(string(@Modified), 1033, 5)"/>
      </td>
     </tr>
    </xsl:if>
   </xsl:if>
  </xsl:for-each>
 </xsl:template>
5. Save the page.  Notice that only the unique values are displayed.  You can use this code in any number of places in the XSLT (for example, inside a drop-down menu).

Let me know what you think!

-John