For your viewing pleasure, some screen captures from the Justice League Unlimited episode "Ultimatum".
A group of young heroes called Ultimen(based on old Super Friends characters like the Wonder Twins) suddenly show up and fight along side the Justice League. Unknown to the members of Ultimen, the are actually clones with implanted memories created by members of Project Cadmus.
When one of the Ultimen, Wind Dragon developes a new power, the teams manager, Maxwell Lord grew concerned and had the heroes undergo testing, hence the shirtless fest below.
It is revealed that the Ultimen are dying and will be replaced by new clones.