I’ve been told about this interesting open source project (article in Spanish) with La Fonera. It consists of an alarm that is activated via an RF emitter (of the kind that are used to open garage doors) and sends an SMS to a mobile number of your choice.

It can be useful for elderly people in need of assistance or shop owners who want to alert someone of a robbery.

The only equipment needed is an RF emitter, a receiver and a Fonera with the OpenWrt firmware.