This is Dan; he can take about 2.5 kilos on his nipples and can be endlessly tortured with different sized weights. His nipples are well stretched and when I first met him I could see his breasts straining underneath his shirt. He introduced me to stretching the nipples using bottles for suction. Great fun and the nipples stand erect for almost a whole session.
My maid Maria has very sensitive nipples. I can pierce, squeeze, tease or nail them and she absolutely adores it. I commanded her to make me an unusual nipple clamp from two pieces of bamboo; I also clamp her tongue when she starts to get a bit lippy.
I find nipples are quite amusing things to torture. I always like to see the effect of a well placed cigarette. The effect is comic: a look of disbelief, horror, pain and anguish comes over your face when I use you as an unofficial ashtray. As I don't smoke myself, I have to keep blowing on the cigarette to keep it alight.
My electric device came with a pair of nipple clamps so I can clamp the nipples and pass an electric current through them at the same time. These clamps are bi-polar so no danger of electric currents running across the chest.
Naturally I've got a lot of nipple clamps ranging from the functional, run-of-the-mill type to really spiteful crocodile clips with very strong springs to snap shut on your poor nipples. These and clothes pegs are great fun but are even better when they're snatched off the nipple.