many home security systems are very expensive for most of peoples but buying some IP Cameras and motion detectors to control your house when you are away through internet is not a bad idea. just mount motion detectors above any entrance like windows (if they have no ) and doors and connect them to a phone (most of motion detectors come with an alerter which can alert you in different ways like alarm or calling a number) so when it detects any motion it will call your mobile phone and then you can control other things with ip cameras and if you see a thief you can call the police

or you can just set motion detectors to call the police straight when they detected any motion. a cheap dlink wired ipcamera costs around 50-60$ so they are not very expensive. so you don't need to pay for monthly monitoring you can monitor your home yourself without any monitoring fee only some knowledge about connecting to a pc remotely is enough.
@sheepdog : actually your solution doesn't need any monitoring and it's a defensive solution too

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Edited by Iniyila, 22 December 2010 - 03:53 PM.