Domoticz is a very light weight home automation system that lets you monitor and configure miscellaneous devices, including lights, switches, various sensors/meters like temperature, rainfall, wind, ultraviolet (UV) radiation, electricity usage/production, gas consumption, water consumption and many more. Notifications/alerts can be sent to any mobile device.

Best of all, Domoticz is open source and completely free! You only need to invest in hardware.

Domoticz is a Home Automation system design to control various devices and receive input from various sensors.

For example this system can be used with:
Light switches,Door sensors,Doorbells, Security devices, Weather sensors like: UV/Rain/Wind Meters, Temperature Sensors, Pulse Meters, Voltage / AD Meters
And more …

System Requirements:
This system is designed to run on most common hardware, this includes:
Raspberry Pi (Model B/2 advised), Cubieboard/Cubietruck, Unix, Apple, Windows
256MB memory recommended, 200MB free hard disk space, Firefox/Chrome/Safari browser. A screen
resolution SXGA = 1280×1024 or higher is recommended. If you want to use Internet Explorer, you need version 10+.

Most information can be found on the Domoticz wiki (