The all new EEW Yarn Counter Version 2 from Dreaming Robots.  The new version has improvements as follows:
 
Magnetic sensor system – The new circuit board uses magnetic sensors instead of an optical sensor. This eliminates issues with yarn fluff blocking the sensor, reduces maintenance, and creates a more robust and reliable system overall.
Improved disk grip – A subtle texture was added to the measuring disk to better grip slippery yarns.
Upgraded clamp design – The new clamp features an articulating head, allowing it to securely attach to a wider variety of surfaces.
Redesigned yarn guides – Smoother yarn flow and fewer tangles during use.
 
Uses:
 
Measure length of yarn to prevent stash waste
Divide yarn in half when knitting socks
Divide yarn when making quilt squares
Split singles in half when plying 2 strand yarn
Track yarn usage as you knit
Weavers sectional warping
 
Features:
Choose yards or meters
Accurate counting (real world margin of error usually under 3%)
1+ year of battery life with typical use (requires 2AA batteries *not included)
Versatile clamping design fits your workflow
Beeps when target length reached
Magnetic case makes changing batteries easy
 
Included:
EEW Yarn Counter
Clamp
Instruction Manual
WPI Card
2 x AA batteries required (these are NOT supplied)