Create a list of installed packages on a Debian based system

#Written by: Michael Wood
#Date 12-01-2011

If you want to view what packages you have installed run:
sudo dpkg --get-selections

To pipe the output to a file run:
sudo dpkg --get-selections > installed-software

Now, say you had a server or your machine setup the way you wanted it.  You can run the above command, save the file, and if you ever have to re-install the operating system you can just issue this command:
sudo dpkg --set-selections < installed-software
sudo deselect

Now all the packages you had installed before will be re-installed very quickly.