How to list available packages in a specific repository on Debian?

Given that the repository you're interested in is in your apt sources, you can find the information on the packages available there in the files apt downloads; for the line

deb http://ftp.de.debian.org/debian jessie main non-free

these would be respectively

• /var/lib/apt/lists/ftp.de.debian.org_debian_dists_jessie_main_binary-amd64_Packages
• /var/lib/apt/lists/ftp.de.debian.org_debian_dists_jessie_non-free_binary-amd64_Packages

(assuming you're on amd64). You can ensure those files are up-to-date first by running

apt-get update

Aptitude can search by archive name:

aptitude search '~Ajessie'

~Aarchive (or, equivalently ?archive(archive)) is documented in the search term reference as

Select packages from the given archive (such as “unstable”).

If you want only the package names, then pass -F %p to format the output appropriately; by default you'll also get status and short description for each package.

Try

wget http://ftp.de.debian.org/debian/dists/jessie/main/binary-amd64/Packages.gz -O jessie.main.binary-amd64.Packages.gz

wget http://ftp.de.debian.org/debian/dists/jessie/non-free/binary-amd64/Packages.gz -O jessie.non-free.binary-amd64.Packages.gz

Followed by

gunzip jessie.main.binary-amd64.Packages.gz jessie.non-free.binary-amd64.Packages.gz

This will give you two files called jessie.main.binary-amd64.Packages and jessie.non-free.binary-amd64.Packages, which will jointly contain a list of packages corresponding to the source line

deb http://ftp.de.debian.org/debian jessie main non-free

As Stephen noted, if the sources entry is in your sources.list, then these files, along with many others, will be automatically downloaded to your disk, along with many others.