This short guide illustrates how to replicate a server, in the sense of how to get the same packages installed. I have tried this on Debian 6.
Save a list of installed packages on the current machine:
cd dpkg --get-selections > selections.txt
Now move selections.txt to the new machine.
Set package list on the new machine and install packages:
cd dpkg --set-selections < selections.txt apt-get update apt-get upgrade
Installed packages can be obtained with:
aptitude search '~i'