If you have unused packages in your system, there is a tool that can help you clean them out.
The gtk orphan package present in the repositories of ubuntu can be conveniently used to clean up your system. It allows you to graphically select and remove packages that are no longer used.
Installation from tar.gz, has the disadvangate that there is no easy way to un-install. But there are two methods of building .deb packages and install them on the system as any .deb package, so the apt-get does the dιrty job of installing and un-installing. You can find them in the article.
A quick how to install packages on ubuntu without internet connection. It can be done in three phases. First a file with needed packages is created. The file is brought to a computer with an internet connection, and the files are downloaded on that computer. Then they are copied to the system that the packages will be installed, and them installation is a piece of cake. (Article in Greek).