Anitya is described as 'A software release monitoring project'. There are three alternatives to Anitya, 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 Anitya are pkgs.org (Free) and whohas (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.