As already mentioned "dpkg -S file" will show which package installed which file. If you install apt-file, you can also search for which package, installed or not, contains a file:

  $ apt-file search /etc/apache2/ports.conf
  apache2.2-common: /etc/apache2/ports.conf
  $ aptitude search apache2.2-common
  p   apache2.2-common    - Apache HTTP Server common files
  $ dpkg -S /etc/apache2/ports.conf
  dpkg-query: no path found matching pattern
    /etc/apache2/ports.conf