Get an 80+ Power supplyOne of the biggest wastes of electricity on the desktop comes from the power supply. Many PC power supplies run at around 60% - 72% efficiency with the rest being dissipated as heat. Since energy costs are very high in many places, running your computer 24/7 can have a noticeable impact on your power bill. By switching to an 80+ approved power supply, your guaranteed to have at least 80% efficiency at 20%, 50%, and 100% power supply usage. This is great news for anyone running their computer as a home server, router, etc... as you can have significant energy usage reductions and lighten up that power bill a little bit.
On a side note...
While reading Jeff Atwood's Coding Horror blog I noticed a very interesting point he made. He makes the point in choosing the right wattage for your power supply. All too often people put in the equivalent of a 500 horsepower engine just because they 'might' need it some day. The problem with that lies in the design of a power supply. They are notoriously inefficient at low power consumption, so while your computer is idling, the power supply is eating the most power it can. So when you are buying your 80+ power supply, please consider how much your rig will need and buy accordingly!
Get an LCD monitor
Many people stare all day at the largest energy monger on their computers. Thats right, your monitor can be voracious consumer of electricity. If you own a CRT monitor, its time to ditch that heavy thing and move to the LCD age. LCD monitors consume roughly one third of the energy a CRT consumes . There are also many other environmental issues you are escaping when switching over to LCD. For instance, CRT monitors have a high amount of lead in them making them difficult to dispose of as well as exposing many people to the effects of lead if disposed of improperly. LCD's have many advantages over CRT's (they weigh so much less!) that its time you consider getting rid of that old CRT (properly, of course) and investing in a shiny new LCD monitor.
Turn on your computers power saving features!
This is a a very simple, very free way to help conserve energy. Simply open up your power settings and pick a few reasonable settings. Have it turn your monitor off after so much inactive time, spin down those disks, put it into hibernate or suspend, just turn the power saving features on!