Target Level in HUD
 

The HUD Level add on puts the target's level up in the HUD so you don't have to look down in the Unit Frames to see the target's level. I find this much easier to find as my attention is mostly on the HUD in the first place, so seeing what level my current target is without having to look to the bottom of the screen is very handy.

The level is at the top of the HUD, just left of the target's power display, across from the target name.

Just unzip this to your WoW Interface/Addons directory and enable it when you log into WoW. It requires nUI, of course :)