Chocolatey!

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+.

Ketarin is a small application which automatically updates and optionally installs setup packages by monitoring web pages for changes. As opposed to other tools, Ketarin is not meant to keep your system up-to-date (like a package manager), but rather to maintain a compilation of all important setup packages which can then be burned to disc or put on a USB stick.

Patch My PC is a small portable application that updates third party applications and Windows updates. It’s critical to keep software up to date to ensure vulnerabilities are patched. There are currently almost 100 third-party applications that Patch My PC can silently update, and the list is growing with each release!

Npackd (pronounced "unpacked") is an application store/package manager/marketplace for applications for Windows. It helps you to find and install software, keep your system up-to-date and uninstall it if no longer necessary.

A command-line installer for Windows.

One of the great things about using open source software is the frequency with which bugs are fixed, security holes are patched, and new features are added. Ubuntu includes a very convenient update manager, which checks the software repositories for updates, and alerts you to install any new versions found. However, if you are annoyed at constantly upgrading, you can follow this guide to tweak the settings of the manager: Navigate to the Software Sources control panel. Under your “System” menu, select “Administration”, and select “Software Sources.” You will be prompted for your root password; once the window opens, select the “Updates” tab: »

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.

OneGet is a unified interface to package management systems and aims to make Software Discovery, Installation and Inventory (SDII) work via a common set of cmdlets (and eventually a set of APIs). Regardless of the installation technology underneath, users can use these common cmdlets to install/uninstall packages, add/remove/query package repositories, and query a system for the software installed. Included in this CTP is a prototype implementation of a Chocolatey -compatible package manager that can install existing Chocolatey packages. »

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.