If your gym has twin cable stations, the standing cable fly is a great option for isolating the chest. Through transverse flexion of the shoulder, the two muscle heads of the chest are primarily targeted, with assistance from the front (anterior) deltoid head (i.e. the front of the shoulder).
If performed correctly, the standing cable fly has a similar action to the lying dumbbell fly, yet has the benefit of continuous tension and resistance throughout the exercise. To explain further; the dumbbell fly is most effective in the lower segment of the arc, as the athlete is overcoming gravity. As the dumbbells are brought up higher through the arc much of the gravitational resistance is lost down through the arms, acting as secondary stabilisers, not the chest.