Probably a recent version of GNU sort. AFAIK "sort -h" dates at least as far back as the version shipped with Debian old-stable.

It can also be achieved with some Perl:

    du -h | perl -e 'sub h{%h=(K=>10,M=>20,G=>30);($n,$u)=shift=~/([0-9.]+)(\D)/; return $n*2**$h{$u}}print sort{h($b)<=>h($a)}<>;'

Source: http://serverfault.com/a/62422/76878

set BLOCKSIZE to some power of 1024 when running du

That's BSD's sort, not GNU sort, isn't it?