monline

Aptitude (an APT front-end) has long been able to distinguish between packages as being automatically installed (based on some dependancy that another package required) or "manually" installed. Since Debian Lenny (5.0) this capability is integrated into the APT package management system and most probably Aptitude's functionality got a lot thinner (since it doesn't have to implement something that's already in APT's core).
I could not find a method to do this (ie. list the manual/auto installed status of packages) the APT-way, but Aptitude's search function does it well enough.
Here's the command to list automatically installed packages:
And the trivial opposite (list of manually installed packages):

Feisty got its End-Of-Life statement and it also means that the official Feisty repository was moved from http://archive.ubuntu.com/ to http://old-releases.ubuntu.com/. If you don't know what's going on and just run apt-get update, you'll see error messages like this ...

The basic method (I mean with default tools on a Debian system) is to use apt-cache:
However I've found that this does not always give correct results.

You can find the details here.

You can find the answer in this post, but I also document it here so I'll have a permanent copy.