Setting your computer to standby after it's been idle for a few minutes will save energy when you're away from your desk and, as an added benefit, it may startle you out of a daydream if you're zoning out at your desk.
The energy savings are impressive. By setting my computer to "sleep" after five minutes of being idle, and putting it to sleep before leaving for the night, I used about 30 percent less energy during the day (from 0.45 kWh to 0.32 kWh) and halved my computer's energy use during the night (from 0.08kWh to 0.04 kWh).
I know, I know, why should my computer use any power at all at night? Well, I don't turn off the computer at night because I hate waiting for the system to boot in the morning. It's embarrassing to admit, but I started the habit back when I had an older computer that took a significant amount of time to start from being off. Now that I have a faster computer, I'll nix the old habit (starting now). (The Department of Energy suggests that you should turn off your computer if you're going to leave it for two hours, but turning it off even if you're only leaving for an hour is probably a good idea.)
In fact, you might want to unplug it altogether. Most computers, like many other appliances, use power to stay in a "ready" mode, even when they aren't on. This is what people are talking about when they refer to a "phantom load." The amount of power used by your computer when it's off but plugged in is small. We're talking watts, not kilowatts.
The folks over at the Lawrence Berkley National Lab drew up this chart of the phantom load (expressed in Watts) used by some common appliances:
I've seen a few studies and articles that talk about re-engineering household electronics to use less than one watt while in standby mode, but in the meantime pulling the plug on your idle appliances is the only way to reduce the waste.