pkgs.org is described as 'The pkgs.org is created to provide you with the simplest method of searching and downloading the newest versions of the Linux packages without the usual excessive popups or spyware'. There are three alternatives to pkgs.org, not only websites but also apps for Linux. The best alternative is Repology, which is both free and Open Source. Other great sites and apps similar to pkgs.org are whohas (Free, Open Source) and Anitya (Free, Open Source).
whohas is a command line tool that allows querying several package lists at once - currently supported are Arch, Debian, Fedora, Gentoo, Mandriva, openSUSE, Slackware (and linuxpackages.net), Source Mage, Ubuntu, FreeBSD, NetBSD, OpenBSD, Fink, MacPorts and Cygwin.