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

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

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)