There are a number of these types of devices on the market at the moment but I chose the ENVI 128 on the basis that it will output the data as a raw XML feed over the USB serial cable (sold separately) allowing me to write my own software but this also means there is a large amount of software available on the Internet that others have created so you're not stuck with the manufacturers package. This is very useful if you use Linux like me and often get overlooked by the vendors.
It was installed very quickly and worked first time, there is a cost calculator as the name suggests that flicks between daily and monthly costs. There's a real-time wattage display, a graph showing the distribution of yesterday's usage over the day, a monthly watt-hour reading and a clock and thermometer which is about as much as you can expect. There is support for individual appliance readers too using plug-in devices sold separately (but not available at time of writing).
My only gripes so far is the quality of the build materials, the plastic casing is cheap like the kind used in the vacuum moulding machine they had back in my school days, easily broken if not left alone and the LCD display is obviously cheap but it's not easily apparent when powered up. It's also mains powered which is a plus in the sense there's no battery overheard (both environmentally and financially) and a negative in the sense it restricts where you can place it around the house, that said the power-cable is fairly generous in length.
Provided you don't expect it to do too much and lets face it - it reads and reports on power usage and that's it, then it does the job and you too can micro-manage your home electricity bill and shout at your family for leaving the lights on in another room you can't even see!