list all packages from a repository in ubuntu / debian

is there a command to see what packages are available from a certain ppa repository?

Simple:

 grep -h -P -o "^Package: \K.*" /var/lib/apt/lists/ppa.launchpad.net_*_Packages | sort -u

Or more flexible:

grep-dctrl -sPackage . /var/lib/apt/lists/ppa.launchpad.net_*_Packages 

For fancier querying, use apt-cache policy and aptitude as described here:

aptitude search '~O LP-PPA-gstreamer-developers'

grep Package /var/lib/apt/lists/(repo name)_Packages

I don't know if this is what you're looking for:

https://superuser.com/questions/132346/find-packages-installed-from-a-certain-repository-with-aptitude

Like it says, Synaptic Package Manager allows you to search by "origin". This isn't programmatic, but it should give you what you're looking for.