An Efergy energy monitor is a wireless device with an LCD screen that shows how much power your household is using. The device is useful for helping reduce power usage by determining what appliances cause the largest power draw.
Nathaniel Elijah has posted on his blog about how he was able to create an RTL-SDR based decoder for the FSK data transmissions sent out by the Efergy energy monitor. His program receives and decodes the transmissions, saving the data to an excel file for later analysis.
Gough Lui from the Goughs Techzone blog has also attempted decoding these energy monitors as well, and has written a large writeup on his experiences.