The Tower of Pain/Nipple Play
I love regular pincer nipple clamps with the chain tied in a knot or two to shorten and apply a little added tension. Pinch them with the tips of my nipples sticking out the end and use a nice vibrator on the exposed ends and you've got me where you want me!
Try vice-grip plyers for a fantastc feeling. The weight hanging down is great also.
Office supplies.
The little black paper clamp binder things work great on nipples. You can bend them to desired tension if you can't take the full pinch. Connecting them with a rubber band gives a similar feeling as the tower of pain although the nipples are pulled toward each other rather than away from the body. On the plus side, they don't stick out and don't really get in the way and the rubber band provides constant tension.