In older versions, (1.7.10 & earlier) it is NEI as a stand alone that also adds the recipe index and some cheatier features. In newer versions there are several options. NEI is now an add on for JEI and just adds the old NEI features that JEI doesn't. I'm seeing the feature in another mod that mostly does other things (PrimalCore, maybe? Dynamic Surroundings? I just got done making my first pass through ~100 config files for a new personal pack). There are also a few small stand alone mods that just add one or more of the overlays, the other main overlay being the chunk borders. At least one of the alternatives uses red & green numbers instead of the Xs. More Overlays and Light Level Overlay Reloaded are the options I can think of.
Thanks, I did see the one with the numbers but found that displeasing to look at while simple Xs look better for me, I just plunk a torch down and move it until the X go away so I don't need to know whether it's 8 or 10 xD After giving it a look "More Overlays" seems like the one, thanks for the help!