Wire

Wire has its own placement layer, so it runs through blocks, walls, furniture, plants and water without displacing any of it, and it hangs in open air with no support. Explosions ignore it entirely. The only thing that removes it is a Wire Cutter, which means a bad run is never destructive, just tedious to undo.

Placed wire is invisible unless you are holding something mechanical. Once a build has a few hundred tiles of it, tracing a fault by juggling a wrench is miserable. A Mechanical Lens fixes that, and The Grand Design goes further by letting you switch individual colors off so you can look at one circuit at a time. Get one before you attempt anything with more than a couple of triggers.

Each wrench color lays an independent circuit, and different colors can share the same tile without talking to each other. That is what makes compact wiring possible: four separate systems can run down the same one-tile shaft and nothing crosses over.

None of it can be placed until Skeletron is down and the Mechanic is freed. Naturally generated wire exists before then in Cavern traps, the Dungeon and the Temple, and you can pull some out of Shimmer, but without a wrench it sits in your inventory. What you can do early is craft timers and pumps and splice them into trap circuits that already exist.

One use people miss entirely: wire an ordinary chest and it becomes a collector. Pulse it and loose items nearby get pulled in and stored.