Lapis and quartz lamps.

Both of these blocks are from the mod Random Things, but they, in my opinion would do well in vanilla for aesthetic reasons.

Lapis Lamp: This lamp illuminates with "light," which can be seen by the player, but does not stop things that require dark, such as mob spawning.

Use: You have a mob spawner and would like to see what's going on inside a little bit better.

Quartz lamp: The exact opposite. It does prevent mob spawning but does not glow in any way that the player can see.

Use: You want to build a dark cave (or any other instance where you want it to be dark for immersive reasons), but don't want to have to deal with mobs in your build, and slabs don't really fit.

Crafting: They are crafted with four of the material surrounding a redstone lamp.

Source