Energy cost money, so it is good idea to keep track on usage of it. Following project is dedicated to monitoring of whole house energy consumption. It has plenty features that enable do various fun stuff like seeing real power usage, apparent power, power factor, RMS voltage/current, frequency and cumulative kWh.
The values can be viewed on four 7 segment LED panel or via web interface that allows graphing energy usage. Energy measurement and Ethernet interfacing is done by Arduino boards. So there is not much of board making than wiring and burning sketches. And once again, be careful when dealing with mains.