Now that I am a runner I don't know how anyone runs without doing yoga as well. The stiffness, tight shortening muscles.... yoga saves the day!
Many poses are helpful, such as Legs Up a Wall, Downward Facing Dog, Reclining Hand to Toe, just to name a few for stretching. Then there's the strenthening poses like Dolphin and Plank, which also work your core to avoid lower back injuries.
Besides runners, I've taught skaters and ballet instructors, who have all benefitted from doing yoga. Along with a great stretch, strenghtening and helping to avoid injuries, yoga is great for concentration as well as proper breathing. When your chest is more open you have more of your lungs available for more air intake. When you're running your heart out... who wouldn't want a little more air?