I use Smappee and its brilliant.
http://www.smappee.com/uk/energy-monitor
It has the extra features of being able to identify your major power-consuming appliances individually so you can see where your electricity pounds are being spent. Also it has a built-in 433Mhz controller so it makes a very cost-effective way of on/off controlling of various appliances up to 1kW using compatible 433MHz plugs (around £10-15 a time, rather than the £30+ of a WeMo or the £40+ of SmartThings Outlet) … just bear in mind that it cannot directly monitor the state of the switches, but it can tell you the power being drawn if it has a high enough wattage to be detected via the current clamp.
I have written a DTH that allows me to poll the Smappee power consumption direct from ST via my LAN, and it has a great IFTTT integration too.