Using Raspberry Pi as hub that talks over wireless (nrf24l01) to networked Arduino based nodes that control appliances. The system allows control and monitoring over the Internet and through instant messaging.

For now, im just saving a list of links that I keep revisiting here
Measure Vcc by analogRead(internal1.1Vref)
Maniacbug RF24l01 Low power node
Raspberry PI to Arduino over Serial

Solar water disinfectant.

This system indicates when water that is exposed to sunlight is safe to drink. {1 pixel LCD + a tiny solar panel + a PIC 10F222 + a rechargeable battery}. The solar panel acts as the transducer that measures sunlight and also charges the battery. The microcontroller measures sunlight and triggers the LCD after sufficient exposure.

Here is a report I wrote many years ago about this project.

{Arduino + a 128x32 OLED display + a JY-MCU Bluetooth module + a LiPo battery}.