I've been using a Kill-A-Watt meter these past few weeks. This is one gadget everyone needs to get.
You basically plug it in between the outlet (or strip) and to the device you want to measure. It measures the load and gives you good metering options. For example, you can plug it in and run it a few hours to days to see the average kWH consumption of a particular device. Multiply that by your energy rates and you can pretty much sum up how much a device would cost you to run monthly. With tons of gadgets and electronics, the electricity bill can get pretty high so this device helps you reign in on those costs.
You can get one at Amazon (here) or NewEgg (here). The hundreds of 4.5-5 stars from users is enough to tell you this is a decently good product.
I was pleasantly surprise to find out which gadgets were consuming too much or too little power. For example, leaving my various AirPlay speakers had little to negligible power consumption at 2-4 watts on standby. Leaving a few RAID and external drive enclosures plugged into the power strip while being unused by a powered down computer yield 25-30 watt of power.
I definitely changed some of my usage habits after using a few of these Kill-A-Watt meters around the house. For example, I had an older AMD Athlon machine running as a ESXi server running a DVR for my cameras. It was also serving iTunes and Plex Media server. At idle, it was still running at 85 watts and easily ramp up to 110-130 watts on load. This was a bit costly to run trivial things 24x7. Hence, I switched to running to an older unused core i5 laptop and my average use is now 20 watts for those same services.
I highly recommend a device like this. Just read the hundreds of customer reviews. Sure, there will be a few lemons here and there but the general consensus is that these pretty much pay for themselves.