Most of the time I have more than one computer that I use. One problem is to get all installed applications synchronized. This is quite useful if you develop scripts which need special applications to run and you like to have them available on every computer you use. So from time to time I synchronize my /etc/apt/source.list followed by:
apt-get clean all
apt-get update
apt-get dist-upgrade
The next step is to get a list of all installed applications on the current local computer. This can be done with the following commands:
dpkg --get-selections "\*" > /tmp/deb-list.txt
Now I distribute the generated file from the previous run by copying it to my second box. On my second box I execute:
dpkg --set-selections < /tmp/deb-list.txt
apt-get dselect-upgrade
The result is that all applications are now synchronized between my first and my second box.