Managed Metadata Web Part

update - 14/07/2011
I forgot to mention that you have to deploy the .wsp as a farm solution.
And another advice: if you are using this web part in an Enterprise Wiki site and you want to show the wiki categories, set the following properties like this:

• ListName: pages
• FieldName: Wiki_x0020_Page_x0020_Categories

update - 05/06/2011
The solution (.wsp) and source code are now available for download.

There is en excellent CodePlex project called SharePoint 2010 Managed Metadata WebPart (http://metadatawebpart.codeplex.com/.) It's a web part that displays a navigation tree based on a managed metadata column assigned to a list/library. I modified it a bit and now it works exactly as I need:

The changes I made are as follows:

• link and item count are displayed only on terms that have items associated with them
• the link supports variables such as term and list GUIDs, this way you can link to the Catergories page and dynamically get all the items at the specified category
• a nice tag icon precedes the term
• the web part now supports RTL languages (such as Hebrew)
• added a fix to support internal field names (http://metadatawebpart.codeplex.com/discussions/220164)

The changes are in 2 places only: the web part code (Taxonomy_WebPart.cs) and the XSL you insert in the XSL String web part property. Here's what you need to change in the original project in order to get the same results:

Web part code - Taxonomy_WebPart.cs

I made some changes to the code to insert the variables for the URL.
Find this comment:

// get our term hierarchy

This should appear afterwards:

// get our term hierarchy
xmlOutput += "" + termSet.Name + "";
xmlOutput += "" + myWeb.Url + "";
xmlOutput += "" + myList.ID.ToString() + "";
xmlOutput += "" + FieldName + "";
xmlOutput += "";
GetRootTerms(termSet);
xmlOutput += "";

Find this method:

private void outputTerm(Term t)

This should be its content:

private void outputTerm(Term t)
{
    xmlOutput += "<term>";
    xmlOutput += "<name>" + t.Name + "</name>";
    xmlOutput += "<rootTermID>" + rootTermCount.ToString() + "</rootTermID>";
    xmlOutput += "<itemcount>" + GetTermCount(t.Id) + "</itemcount>";
    xmlOutput += "<termGuid>" + t.Id.ToString() + "</termGuid>";
    if (t.Terms.Count > 0 && CurrDepth < Depth - 1)
    {
        GetTerms(t);
    }
    xmlOutput += "</term>";
}

Here's a sample of the XML this code generates:

<termset>
    <name>Wiki Categories</name>
    <webUrl>http://server/Wiki</webUrl>
    <listId>102afb60-f8fc-4755-96ff-e2045da851ba</listId>
    <fieldName>Wiki_x0020_Page_x0020_Categories</fieldName>
    <terms>
        <term>
            <name>Data Bases</name>
            <rootTermID>0</rootTermID>
            <itemcount>0</itemcount>
            <termGuid>2947210c-bb52-48b7-8511-212b25658b08</termGuid>
            <term>
                <name>SQL Server</name>
                <rootTermID>0</rootTermID>
                <itemcount>0</itemcount>
                <termGuid>08daa347-a5df-4f6f-9210-52a394ee6ae5</termGuid>
            </term>
            <term>
                <name>Oracle</name>
                <rootTermID>0</rootTermID>
                <itemcount>0</itemcount>
                <termGuid>c0bca60f-1bca-42a8-8f95-e1b37d56591d</termGuid>
            </term>
        </term>
        <term>
            <name>Operating Systems</name>
            <rootTermID>1</rootTermID>
            <itemcount>0</itemcount>
            <termGuid>f15f26ce-dd87-41bd-be25-2ca74e0a9085</termGuid>
            <term>
                <name>Windows Server 2008 R2</name>
                <rootTermID>1</rootTermID>
                <itemcount>1</itemcount>
                <termGuid>34341dc2-00b3-4e04-b6ee-b06adc2efaec</termGuid>
            </term>
            <term>
                <name>Ubuntu</name>
                <rootTermID>1</rootTermID>
                <itemcount>0</itemcount>
                <termGuid>3ab60c75-c74b-4e2e-b604-769551a1face</termGuid>
            </term>
        </term>
    </terms>
</termset>

I also incorporated this fix.

XSL

Now I modified the XSL to reflect the changes in the XML structure, and to design it a little. You can replace the value of the variable _xslString in the web part code with this snippet, or you can copy and paste it in the XSL String web part property (in the web part directly in the page.)

<?xml version='1.0' encoding='utf-8'?>
<xsl:stylesheet version='1.0' xmlns:xsl='http://www.w3.org/1999/XSL/Transform' xmlns:msxsl='urn:schemas-microsoft-com:xslt' exclude-result-prefixes='msxsl'>
    <xsl:output method='html' indent='yes'/>
    <xsl:template name='terms' match='//termset'>
        <xsl:param name='d'/>
        <xsl:variable name='webUrl' select='/termset/webUrl'/>
        <xsl:variable name='listId' select='/termset/listId'/>
        <xsl:variable name='fieldName' select='/termset/fieldName'/>
        <div id='accordion'>
            <xsl:for-each select='terms/term'>
                <xsl:call-template name='termtemplate'>
                    <xsl:with-param name='webUrl' select='$webUrl' />
                    <xsl:with-param name='listId' select='$listId' />
                    <xsl:with-param name='fieldName' select='$fieldName' />
                </xsl:call-template>
            </xsl:for-each>
        </div>
    </xsl:template>
    <xsl:template name ='termtemplate'>
        <xsl:param name='webUrl' />
        <xsl:param name='listId' />
        <xsl:param name='fieldName' />
        <xsl:element name='div'>
            <xsl:attribute name='style'>
                text-indent:<xsl:value-of select='count(ancestor::*)*20'/>px;
                padding-top: 2px;
            </xsl:attribute>
            <img alt='' src='/_layouts/Images/EMMTerm.png' style='vertical-align: middle;' />&#160;
            <xsl:choose>
                <xsl:when test='itemcount != 0'>
                    <a href='{$webUrl}/_layouts/Categories.aspx?FieldName={$fieldName}&amp;FieldValue={termGuid}&amp;ListId={$listId}'>
                        <xsl:value-of select='name'/>
                    </a>&#160;(<xsl:value-of select='itemcount'/>)
                </xsl:when>
                <xsl:otherwise>
                    <xsl:value-of select='name'/>
                </xsl:otherwise>
            </xsl:choose>
            <xsl:for-each select='term'>
                <xsl:call-template name='termtemplate'>
                    <xsl:with-param name='webUrl' select='$webUrl' />
                    <xsl:with-param name='listId' select='$listId' />
                    <xsl:with-param name='fieldName' select='$fieldName' />
                </xsl:call-template>
            </xsl:for-each>
        </xsl:element>
    </xsl:template>
</xsl:stylesheet>

Note that the terms are linked dynamically to the Categories page, the URL is populated with the necessary query string parameters:

http://<site address>/_layouts/Categories.aspx?FieldName=<field internal name>&FieldValue=<term id>&ListId=<list id>

The Categories page after a click on 'North America':

Conclusion

Now all you have to do is rebuild, repack and deploy. Enjoy!