Ubuntu 14.04 and above

The apt tool on Ubuntu 14.04 and above makes this very easy.

apt list --installed

Older Versions

To get a list of packages installed locally do this in your terminal:

dpkg --get-selections | grep -v deinstall

^{(The -v tag "inverts" grep to return non-matching lines)}

To get a list of a specific package installed:

dpkg --get-selections | grep postgres

To save that list to a text file called packages on your desktop do this in your terminal:

dpkg --get-selections | grep -v deinstall > ~/Desktop/packages

Alternatively, simply use

dpkg -l

(you don't need to run any of these commands as the superuser, so no sudo or any other variants necessary here)

This script can list all packages from a particular server/branch (modifying the two declared variables):

#!/bin/bash

server="http://it.archive.ubuntu.com/ubuntu/"
branch="maverick-updates/main"

apt-cache policy $(dpkg -l | awk 'NR >= 6 { print $2 }') |
  awk -v server="$server" -v branch="$branch" \
    '/^[^ ]/      { split($1, a, ":"); pkg = a[1] }
    nextline == 1 { nextline = 0; if ($2 == server && $3 == branch) print pkg }
    /\*\*\*/      { nextline = 1 }'

if you need to list all packages not coming from a particular server, independently from the branch, replace the next-to-last line with:

nextline == 1 { nextline = 0; if ($2 != server) print pkg }