If you are running Debian and have lost track of which packages you are running, it could be useful to get a backup of your currently installed packages. You can get a list by running:
dpkg --get-selections > debianlist.txt
This will put the entire list in debianlist.txt. You could then install the same packages on a different computer with:
dpkg --set-selections < debianlist.txt
You should bear in mind that you would also need to copy over configuration files from /etc when copying your system to a new computer.
To actually install the selections, use:
apt-get -u dselect-upgrade