Fhem is a GPL'd perl server for house automation. It is used to automate some common tasks in the household like switching lamps / shutters / heating / etc. and to log events like temperature/humidity/power consumption.
The program runs as a server, you can control it via web-frontends, telnet, command line program or TCP/IP directly.
In order to use fhem you'll need a 24/7 server (PC, MacMini, Fritz!Box, NAS, etc) with a perl interpreter and some hardware to access the devices.