This stretch is, in my opinion, The King. The reason I hold this opinion so strongly is that it stretches areas that I tend to have tightness in, especially if I've been running a lot. I think that all of the years doing heavy squats and deadlifts in my bodybuilding/powerlifting competition days created some shortened muscles in my hips and lower back. Oh well, live and learn, right? Always!
Anyway, to get specific about this stretch, what you're doing is squatting down as low as you can to mostly sit on your heels. Then, with your hands on the ground in front of you and your elbows between your knees, start pressing a little outward with your elbows and experimenting with pressing your upper body forward. Sometimes I even stretch my upper body so forward that I put the top of my head either on the ground or on a wall in front of me. For those of you who do yoga, you'll feel like you're in the beginning stages of the Crow Pose, but I actually get way more out of this pose than the full crow pose because of the awesome stretch on my lower back muscles. It's not the most graceful-feeling pose, but it sure is effective!
I'll be posting some more post-run stretches in the coming few days.