Windows/Linux: Sleeping your computer is a useful way to save energy, but free utility SonarPM takes power management an awesome step further: It puts your computer to sleep automatically when you walk away from your desk — using sonar.
Almost any computer with speakers and a microphone can take advantage of this software, so most laptops are good to go. It works by emitting an inaudible frequency from your speakers, and detecting echoes with your microphone, determining whether you are sitting at your desk or not. If you get up, your computer will see that you have done so and automatically put your computer to sleep. Sure, you could always just use a hotkey, but this requires literally no energy on your part — and it's sure to be a heck of a conversation starter.
The program is open source, so if you've got some tinkering skills, you can make it do all sorts of things when you walk away (as described in the video above by Tinkernut). If you just want the sleep functionality, you can still download and install the package and use it as-is (if you're a Windows user, that is — Linux users need to compile from source — check the instructions on the site if you need help doing this). It doesn't necessarily work with every microphone and speaker equipped computer, so test it out to see if it does with yours — you may need to calibrate it a few times before it works well (it took me a few tries and a little bit of waiting afterward before it started working in Windows on my MacBook Pro).
Sonar Power Manager is a free download for Windows and Linux systems. Sleeping your computer whenever you walk away seems a little heavy handed (do I really want to put my computer to sleep when I'm going to get a glass of water?), but the possibilities are endless, and if you are interested in bending it to your will, Tinkernut's guide is an excellent place to start.