The Weirding Gadget

The Weirding Gadget is a mod by AtomicBlom that adds a single block of the same name.

Weirding Gadget
The Weirding Gadget is the sole block added by The Weirding Gadget. When placed, it will continually keep loaded a 5x5 chunks area centered on itself. When the player that placed it logs off the server, the Gadget will spin slower and slower, until it stops after 7 days, at which point it will no longer keep chunks loaded. When said player logs back on the server, the gadget reactivates itself and its countdown is reset.

Any other player can right click the Gadget, resetting its countdown and adding themselves in the "tracked" list of the Gadget. This means that the Gadget will only start counting down when all tracked players are logged off, and will reset when any of them logs back.