my $last;
my $i = 0;
while (<IN>) {
   chomp;
   $i++ if (!defined($last) || $_ ne $last);
   $last = $_;
   print OUT ("$_ $i\n");
}
[download]

One liner:

perl -lpe "$i++ if ($_ ne $last); $last = $_; $_.=' '.$i;" inputfile >
+ outputfile
[download]

(Swap ' for " and vice-versa if you're not using Windows.)

Update: eek, that's what I get for not testing. Added the missing $last=$_.