Roughly Enough Resources
In short, it will show you what blocks spawn at what heights in the world. Note that there are some cases where blocks (which are what the world is made of) and items (which is what REI will show recipes for) don't completely mesh up. RER tries to account for this in several ways. Some of these will show up as multiple recipes attached to the same item -- for example, torches on walls and torches on the floor are different blocks, but the same item. Others will show as a vaugely related item -- water and lava show as their corresponding buckets, and fire blocks show up as flint & steel. A few different blocks show up as a question mark. but you can get the name of the block by hovering over it. REI, somewhat sadly, has strangely partial support for air -- if you try hitting the "R" button on an air item, nothing will happen. Recipes for air are only visible on the list of all "world generation" recipes.