I've been experimenting with a new version of the Strain Wave Drive (same idea as a harmonic drive but not made by the Harmonic drive company). It is fully functional and includes a dynamic spline for the output rather than the stabilizer and cup style flex spline as in the previous prototype.  This version still uses bbs for the bearings, which is probably a weak point in the design but keeps the cost of the drive below $5.  The double ring bearings on the dynamic spline help to stabilize the output of the drive and reduce any off axis wobble.   Video of the new strain wave drive in action.