If you're installing Debian and using preseeding to automate things, there are a lot of settings that you can change directly during the installation phase. Most of the useful things you might want to change are those for the installer itself (e.g. choice of disk partition layout, keyboard, language etc.). However, it is also possible to configure things up in a lot of other packages too if you know the appropriate runes. Finding out exactly what settings are possible can take quite a lot of effort.
Here is some help with that. I've written a script to extract all of the debconf templates in the Debian archive for each of the following releases, pulling out all the places where a template reflects a question or choice. Not all of these settings will make sense out of context, and some of them may not be sensible to use in a preseed. YOU HAVE BEEN WARNED! But I'm hoping this comprehensive listing might be useful for some people regardless.
I've only generated the set for amd64; other arches should be almost identical.
In case you'd like to do this yourself, my script is here.
Steve McIntyre <firstname.lastname@example.org>