You might prefer to have a clean system on reinstall but sometimes it is nice to reinstall applications from a previous machine/setup. Keeping a backup list of packages will make this a snap. Just give your package manager a list of all the packages you want it to install and let it rip.
Here are the backup and restore methods for each of the major distros/package managers. Backup
dpkg –get-selections > installed-software.log
dpkg –set-selections deselect install