One of the oldest and most efficient (zero backlash) is the Oldham coupling
http://www.mechanisms101.com/oldham.shtml
They start at about $8 in the Misumi catalogue.
I have found that applications that use aluminum Helical with in any kind of medium/high torque servo, they usually end up fracturing due to metal fatigue, and end up putting on a Stainless version.
Al.