I have recently used this very neat trick within the dpkg packet manager. Have you ever started a fresh install, and forgot which applications you had installed on your old installation? It's a pain. Even when you think you have them all installed, you go to do something, and realize Damn! I forgot to install that.
If you load up a blank terminal, and run the following command:
dpkg --get-selections > apps.txt
This will store a list of every application that is currently installed to a file called apps.txt. If you are wanting to re-install your OS, then make sure that you save this file to a USB stick!
Once you are ready to install all your applications again:
dpkg --set-selections < apps.txt
apt-get dselect-upgrade show
Now after a lot of downloading and installing, you will once have all of your applications installed again. It may be an idea before you re-install your OS to back up the /etc/apt/sources.list file, so that you are able to download the applications that you may of got else where.
If you have any questions about this methord, then please leave a comment!