This pose is harder than it looks, but it offers a nice deep stretch for the hip flexors, quads, and even the chest.
How to do it: Begin kneeling, with the roller perpendicular to you on the floor. Step your right foot forward and over the roller, lunging into your leg, shifting your weight off of your back knee. Position the roller under your left thigh in front of your knee, tucking your toes under to lift up into a low lunge (just be sure to avoid putting pressure on top of your kneecap by positioning the roller in front of your knee at all times). If this is a deep stretch for you already, simply stay here and hold this position.