Portable Software Center (PSC) is an application that lets you create a "portable" local repository (works with Debian-based systems, tested on Ubuntu) to which you can add any packages you want, with the purpose of installing them later on a computer that's not connected to the Internet.

With PSC you can add packages to your custom local repo from any repository enabled on your computer and it automatically downloads the dependencies for each selected package. You can even add .deb packages which you've manually downloaded. Then, you can copy the Portable Software Center folder to other computers that aren't connected to the Internet and install the packages you've added to your repository.