PrimePower is popular tool to estimate power. Almost all the PD tools have some power estimation commands. Apache is more for IR drop and power integrity...it has its own modeling views called apl libraries while Primepower and other physical design tools (Encounter etc) use .libs to estimate the power numbers.
power integrity : means the power supply doesn't have unwanted droop when the chip is functional. It refers to checks which take VDD supply remains within a certain range for the chip to functional.
It involves adding decaps in the regions of the chip where there is a lot current drawn from the power supply.