The mod was originally created by 4poc (in minecraft 1.6.4 and earlier). http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1284971-light-level-overlay
Since he did not update the mod for a long time, I decided to re-implement it. So I made a mod called “Light Level Overlay Reloaded”.
The default shortcut key to turn on/off light level overlay is F4. You can change it in “Options -> Controls”.
You can use Shift + F4 to toggle sky light calculation. Ctrl + F4 to toggle overlay type.
You are welcome to include this mod in modpacks as long as you provide credit by linking back to this page.