Incomplete list of downstreams including dpkg in their distribution (which are not tracked currently by the PTS). Interesting changes found on their packages might include portability and bug fixes.
Debian derivatives:
GNU/Linux:
(1.16.x) Alpine Linux
(1.18.x) ALT Linux
(1.17.x) Arch Linux
(1.17.x) Fedora
(1.18.x) Gentoo
(1.13.x) GoboLinux
(1.17.x) Mageia
(1.18.x) OpenEmbedded
(1.18.x) OpenSUSE
(1.16.x) PLD Linux
(1.16.x) Slackware
(1.17.x) Source Mage
(1.17.x) UHU-Linux
(1.18.x) Yocto Project
BSD:
Solaris:
(1.18.x) OS Dyson
Darwin:
See also the output of whohas dpkg.