MacLibre!

Chocolatey NuGet is a Machine Package Manager, somewhat like apt-get, but built with Windows in mind. It is built on top of NuGet command line and the NuGet infrastructure.In terms of updates, it allows you to update nearly everything on your machine with a few simple keystrokes.

Synaptic is a graphical package management program for apt. It provides the same features as the apt-get command line utility with a GUI front-end based on Gtk+.

Homebrew-cask provides a friendly homebrew-style CLI workflow for the administration of Mac applications distributed as binaries.

The MacPorts Project is an open-source community initiative to design an easy-to-use system for compiling, installing, and upgrading either command-line, X11 or Aqua based open-source software on the Mac OS X operating system.

App Grid is a new lightweight Ubuntu Software Center alternative that tries to make application discovery easier by using a grid view which includes app screenshots and ratings.

What is Fink?

Ubuntu Sources List Generator is a website where you can generate a sources.list file with a lot of useful repositories.

win-get is an installer and software repository for Microsoft Windows, like apt-get in Linux.

The Debgen is a tool that allows the user to create /apt/sources.list file by adding some extra repositories beyond the standards.

PostInstallerF is a small, lightweight, useful tool that can be used to install 100+ essential softwares, Games, audio/video codecs, and third-party repositories which are not included by default in Fedora, Ubuntu and Manjaro.

This program installs a collection of some of the best Ubuntu software and can be considered to be one of the top things to do after installing Ubuntu.

CoApp is an open-source package management system for Windows.