You will need a Google API account (http://api.google.com), as well as the SOAP::Lite(http://www.soaplite.com) and HTML::LinkExtor (http://search.cpan.org/author/GAAS/HTML-Parser/lib/HTML/LinkExtor.pm) Perl modules to run this hack.

Save the code as mindshare_calculator.pl, remembering to replace insertkey here with your Google API key:

#!/usr/bin/perl -w

use strict;
use LWP::Simple;
use HTML::LinkExtor;
use SOAP::Lite;

my $google_key  = 'insert key here';
my $google_wdsl = "GoogleSearch.wsdl";
my $yahoo_dir   = shift || "/Computers_and_Internet/Data_Formats/XML_  _".
                  "eXtensible_Markup_Language_/RSS/News_Aggregators/";

# Download the Yahoo! directory.
my $data = get("http://dir.yahoo.com" . $yahoo_dir) or die $!;

# Create our Google object.
my $google_search = SOAP::Lite->service("file:$google_wdsl");
my %urls; # where we keep our counts and titles.

# Extract all the links and parse 'em.
HTML::LinkExtor->new(\&mindshare)->parse($data);
sub mindshare { # for each link we find...

    my ($tag, %attr) = @_;

    # Continue on only if the tag was a link,
    # and the URL matches Yahoo!'s redirectory.
    return if $tag ne 'a';
    return unless $attr{href} =~ /rds.yahoo/;
    return unless $attr{href} =~ /\*http/;

    # Now get our real URL.
    $attr{href} =~ /\*(http.*)/; my $url = $1;
                $url =~ s/%3A/:/; # turn encoding into legits.

    # And process each URL through Google.
    my $results = $google_search->doGoogleSearch(
                        $google_key, "link:$url", 0, 1,
                        "true", "", "false", "", "", ""
                  ); # wheee, that was easy, guvner.
    $urls{$url} = $results->{estimatedTotalResultsCount};
}

# Now sort and display.
my @sorted_urls = sort { $urls{$b} <=> $urls{$a} } keys %urls;
foreach my $url (@sorted_urls) { print "$urls{$url}: $url\n"; }