Most of them with a new, innovative design. Today's CPUs produce a lot more heat than in the past and overclock much higher. Companies have made heatsinks out of copper, aluminium and even silver. Silver has the best heat dissipation but it is extremely expensive to make a full heatsink out of it.
Copper is a great heat dissipator however, it too is also quite expensive and is easily damaged.
This is the main reason aluminium is the mainstream heatsink metal used today, providing good heat dissipation at a lower cost. When testing, we tried to duplicate the environment that the average user experiences during normal use. First of all, we tested in a closed case with no blowholes. We used Arctic Silver thermal compound with all heatsinks, you can view our comparisson between OCZ Quicksilver and Arctic Silver II thermal compounds here . We thought we would differ from the norm by not only testing with all stock fans, but with 3 other fans aswell.