Revisiting “How Much Power Does My Laptop Really Use”?: A good look at just what uses battery life on your laptop (and your desktop too, I suppose). The author hooked up a Kill A Watt meter, and ran various tests.
The top consumers of your laptop’s power are the CPU, the GPU, the DVD, and WiFi — in that order.
I was surprised about the WiFi — I didn’t expect that to use so much power.