To be clear, the real zlib lives on http://zlib.net/
The .org variant is basically typosquatting the name.
FWIW, zlib.org gives a
<META HTTP-EQUIV="Refresh" content="0; url=http://www.zlib.net">
zlib.org is temporarily redirected to <a href="http://www.zlib.net">zlib.net</a>