The usual, or even classical way is to create the list of installed packages with sudo dpkg --get-selections > package_list, and then restore when/if necessary with cat package_list | xargs sudo apt-get -y install.
As VihangD points out in his serverfault answer, the same can be achieved with aptitude, while also excluding dependent, automatically installed packages (which are included by the classical method). To create the list of packages, run aptitude search -F '%p' '~i!~M' > package_list. Here, -F '%p' asks aptitude to only print package names (instead of the default output, which also contains package state and description); search term ‘~i!~M’ asks for all non-automatically installed packages.
To install packages using the created list, run xargs aptitude --schedule-only install < package_list; aptitude install. The first of these two commands instructs aptitude to mark all the packages from the list as scheduled for installation. The second command actually performs the installation.
Hamish Downer suggests an alternative way of getting the initial package_list: using the deborphan utility, deborphan -a --no-show-section > package_list. This command asks deborphan to show a list of packages, which have no dependencies on them. Sounds very similar to what we did with aptitude above, but using deborphan will most likely result in a much shorter list of packages (on my system, deborphan printed 291 package names, aptitude printed 847, and dpkg printed 3650 package names). One more potentially important difference between aptitude- and deborphan-produced package lists is that aptitude only specifies package architecture when it is different from native (e.g. 'googleearth:i386' on a 64-bit system), while deborphan specifies architectures for all the packages (resulting in e.g. 'google-talkplugin:amd64' and 'googleearth-package:all' on a 64-bit system).
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