Power!

Did you know that the average desktop computer uses over 80 Watts of power while it's on? That;s not counting the monitor. Add another 50W or so for that one. Even when you turn everything off, your monitor, speakers and computer together are still using in excess of 15-20W (and a little more in sleep mode)??
That's why it's a good idea to get a power strip and plug everything in there and shut off the power strip switch after shutting your computer off. You won't just save money, you'll also protect your computer from power surges!
The only downside to doing this is that there will be a greater demand for power from the CMOS (real time clock) battery on the mainboard of your PC. Usually these batteries will last 5-10 years or more if you keep your PC plugged in. However, if you keep it mostly unplugged, they may last a little less than 5 years. Those batteries, typically CR2032 coin cells, are only a few bucks, so it's hardly a real downside.
People still tell me that they think it's better to keep the computer running 24/7. That's a myth that got started when people thought that hard drives would wear quicker when doing multiple starts and stops. That may have been true for the first generations of drives, but it really isn't anymore. Laptop drives actually stop & start repeatedly to save battery power. So unless you actually use your computer for 24 hours in a day, turn it off at night! It will save you about 1.5 kW/day on average! That's about $10/month!

Now if you're really serious about tracking power consumption in your home, you could consider this device: BLUEline PowerCost Monitor and Energy Meter. Even though I do not have personal experience with this particular device, I did create one myself that logs my power use for the entire home. It gave me a lot of insight where I could save some money. On average I've saved over $300/yr in electricity!
Another nice device is the Kill-A-Watt meter. You plug it into the wall and them plug any device (or power strip) into it to get an idea of how much power it uses.