LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 20000 ohms, but when they are illuminated with light resistance drops dramatically.
Can be used to sense Light
Easy to use on Breadboard or Perf Board
Easy to use with Microcontrollers or even with normal Digital/Analog IC
Small, cheap and easily available