Seradiris Vasilis designed SMS control circuit some time ago. It allows to control up to 8 devices through relays. He uses an old Ericsson mobile phone model which is connected to AT90S2313(now Attiny2313).
Circuit is very easy to build as it requires very few components. To activate on of 8 relays you have to send specific SMS message with device number and ON/OFF command. Source code and firmware is available for download. This handy device may be useful in many areas. As author says it may restart Servers, ADSL modems, lock the doors, switch lights or block car engine enywhere without reaching them.