If I need to reinstall a Ubuntu/Debian-based Linux OS (or mirror a software selection to another machine), this is how it can be done. On the source machine:
su
dpkg --get-selections > ~/Package.list
cp -R /etc/apt/sources.list* ~/
apt-key exportall > ~/repository.keys
Then just copy the files to the target machine:
If I install a new computer for myself (or if I do a fresh reinstall on an old machine), I want the computer have the software I am used to. To automatize this process, there a different approaches. If you use Ubuntu, you can use Ubuntu One to syncronize your software selection across many different machines (starting from Ubuntu 11.10 Oneiric Ocelot). If you use something else, it is getting tricky. You can use the following commands to generate a list of installed packages and load the list later for installation:
dpkg --get-selections > installed-packages.lst