Lightchecker

With the Lightchecker you can see where mobs can and cannot spawn.
(http://minecraft.gamepedia.com/Light)

Light Level:
7 - 0 red to black
10 - 8 yellow to orange
14 - 11 white - blue - green

Note:
- This requires OptiFine to work.
- This ignores sunlight level.
- Works with all resolutions.

How to install this pack?
- Press Start and type in %appdata%
- Open .minecraft
- Find and open your .minecraft folder
- Drag and drop or copy and paste the .zip file into the resource pack folder
- Boot Minecraft with OptiFine
- Go to options - > resource packs
- Select Lightchecker. Lightchecker needs to be placed on the first position in the list of activated resource packs.
- Done!

See Monsterley:

Monsterley (32px)

Monsterley HD Universal (128px high-resolution semi-realistic resource pack)