Here's a Perl 6 solution, works with current Rakudo. Not really golfed, just the way you'd write it in a terse way:

$ cat /etc/passwd | perl6 -e '.say for lines.sort: +*.split(":")[2]'
[download]

The nice part is that Perl 6's sort method automatically does the Schwartzian transform for you if the comparison code object takes only one argument.