Daylight Senor! Moonlight Sensor?
Yes you read the topic right there is a Moonlight Sensor. When you place the Daylight Sensor in the world just right click it, and it will change from Daylight to Moonlight. Great for lighting up places without using redstone all over the place. In a cave you can light up Redstone Lamps using this technique for further down mining. If you wish to have a door open at night instead of during the day it is very useful for that too. Also if you want lights to come on during the night you can do that too. I just happen to stumble upon this literally by accident. I was testing something in my Single Player Creative World and placed one to turn something off during the day. I right clicked instead of Shift+Right Click to place a block on top of it and it changed colors. I thought what does that do?! Then I noticed the color looked kinda like moonlight so I did a test with Redstone lamps and guess what it does work with moonlight too!