Doug Larson shows one of my favorite shoulder stretches.
Variation: After Pressing sessions I like to bring the intensity of this stretch up to 11 by using a medicine ball (gives a greater ROM) instead of a foam roller and holding a 5kg in each hand (roughly 3x 30-45s).
Check out Mobility 101 for more stretching ideas.
Also check out the Zygmunt Smalcerz equivalent of this stretch.
Update: Here is Apti Aukhadov doing basically the same thing with one of his seminar attendees.