Light Monitor

Light Monitor is a small mod that displays information about light level in the top left corner of your screen. The Information provided lets you determine if mobs will spawn, what plants will grow and information on if you are standing in a slime chunk.

As with most mods, Modloader will be needed to install:
http://www.minecraftforum.net/topic/75440-v11-risugamis-mods-everything-updated/

And here is the link to the mod:
http://www.planetminecraft.com/mod/light-monitor---in-game-light-level-display---173/

You can also have your light monitor present now by pressing Shift+F3 and looking to your 'bl', 'sl' and 'rl' values.

bl Shows the light level from other blocks besides the sun and moon (blocklight).
sl Shows the light level from the sun and moon (skylight).
rl Shows the "raw", total light level to where you are standing (rawlight).