S3 Standby State Done Right
For Earth Day, Cameron Butterfield has written in with a pointer to his article on how to get your Windows PC into S3 sleep, and why you want to. It covers the question of how to take advantage of this extremely low-power mode even when your machine is an "always on" file server, remote desktop, or VNC server.
There's also programs that use the idle thread to put the CPU to sleep when it's not in use: CPUIdle is a good one, even if you do have to pay for it. That's probably a better option if you're running an always-on system like a server.
The higher the technology, the sharper that two-edged sword.