Reduced rpm list?

Hi all.

I'm trying to find a way to generate a list of all RPMS on a system that are not dependencies of other rpms.

Said another way, I want to generate the smallest possible list of RPMs that would re-create the system after all the dependencies are installed.

Said yet another way... If I do rpm -qa on 20 servers and I want to find out what packages are installed on all 20, I don't really care that all 20 have libc.

Does anyone know an rpm or yum command or script that will do that for me?

Thanks!
-Dylan