Extracting Unique Values with XSL By Dave Pawson as found on http://www.bernzilla.com/item.php?id=333 (Bernie Zimmermann's Blog)

subcategories.xml

<Main>
<Level>
<Category id="a">
<SubCategory>one</SubCategory>
<SubCategory>two</SubCategory>
<SubCategory>three</SubCategory>
</Category>
<Category id="b">
<SubCategory>one</SubCategory>
<SubCategory>four</SubCategory>
</Category>
<Category id="c">
<SubCategory>two</SubCategory>
<SubCategory>five</SubCategory>
<SubCategory>six</SubCategory>
</Category>
<Category id="d">
<SubCategory>seven</SubCategory>
</Category>
</Level>
</Main>

unique_values_extract.xsl

<xsl:variable name="unique-listing" select="/Main/Level//SubCategory[not(.=following::SubCategory)]"/>

<ul>
<xsl:for-each select="$unique-listing">
      <li><xsl:value-of select="." /></li>
    </xsl:for-each>
</ul>