Given the importance of reducing standby (OFF) power consumption in many household appliances, here is my best effort so far at a 50/60 Hz zero crossing detector. To be fair all the credit should go to Robert W. Hilsher for his design idea "A Unique Discrete Zero-Crossing Detector" EDN 1999.
In the attached LTSpice schematic, voltage source V1 represents the 115/230VAC line input, D1 to D4 is the bridge rectifier supplying a small flyback SMPS and voltage source V2 represents its auxiliary supply. Note: Resistor R2 is included to load the bridge rectifier so that D1-D4 simulate correctly.
Power consumtion for the cicuit shown is a little over 13mW, 11mW for the AC drive into R1 and about 2.5mW supplied by the SMPS auxiliary supply driving the opto-coupler LED D6 at a peak current of around 1mA. Bench measurements agree well with the simulation and the circuit has been tested at 300VAC line voltage, and all the way down to 20VAC, that's due to the Sanken STR4A164 flyback. Not yet tested for surge or fast transients so some additional protection may be needed. Back end signal processing on the secondary side, opto output, is not shown but adds a few mW.
EDIT: Schematic corrected for correct AC line voltage.