Tensioner

A tensioner is a device that applies force to maintain tension. They are used to reduce slack created in chain drive and belt drive mechanisms.

Shims

design judges HATE him, most structurally sound, most smoothbrain, likely the lightest -assembly +weight -get laughed at by design judges bc they are all biased against it smh my head

Idler

Another sprocket or pulley is used to add tension to the chain or belt. It does not add any torque to the system and freely spins.

A idler chain tensioner used on a bicycle, from http://www.heatsinkpads.com/store/p12/Heatsink_Ultralight_Chain_Tensioner.html

Guide

Chain guides use a durable low friction material to squeeze the chain into tension. This is most commonly used for timing chains (kept lubricated inside the engine) and sometimes on bicycles.

Eccentric

big brain time +rigidity +infinite adjustability if using friction (may slip) -design complexity -limited finite number of tensioning options if using shear pins

Turnbuckle

-play-wear-packaging -rigidity[citation needed] +infinite adjustability can be done w coarse/fine pitch instead of handedness (i completely forgot what the name is for this kind of turnbuckle, but they have less wear/play [citation needed] but allow a smaller range of motion)