Making your computer use less power is a necessary but difficult to implement issue... unless you know where to look and what to do. You want to use less power to conserve battery life of your laptop; you want to use less power on your servers to save on electricity and cooling costs. And you want to use less power on your desktops to save electricity consumption, all resulting in lesser pollution, more savings and in the case of laptops, more productive time on battery.
http://www.lesswatts.org/ is the website dedicated to help you with configuring your system to use less power. The developers are also examining and fixing the code of the worst-offending applications, so configuring your system for using less power (when it really should by default) becomes less and less necessary.
For now, you can check instructions at all the websites available as to what to do with your kernel, CPU, hard disk, GPU (yeah, tune them down as well. We don't do graphics-heavy work to warrant running the GPU at full clock always) among other things. Also, one of the biggest consumers of power, the LCD backlight -- that can be tuned without at configuration at all. You just have to adjust it to the lowest visible best.
However, what's more interesting to me is small things that we can do, like stop cursors from blinking, since they need X to wake up for each time it has to blink. There's a really nice compilation for the popular applications on one page. Go do it, it'll help the CPU remain in an idle state (and hence consume less electricity) longer.