Let's try this game: 1. Pick a favorite block from any mod you've played; 2. Explain what it does and why is it so good; 3. Optionally link relevant images with this block; 4. Don't post the block if you are the one who created it. The goal is to get familiar with cool blocks you possibly haven't seen or used yet. I'll start with the Marker from a mod 'Builder's Guides': This block is intended to be used as a guide for building structures. It can display various 'ghost' shapes for assitance. Here it shows the shape of two walls: There are several modes and shapes, including spheres, cuboids, stairs; they are quite configurable by a tool of this mod (you can rotate, expand/shrink, change color and such). I've used it mostly for outlining range of monster spawners or farms.