Using a min/max analogue logic module as a phase comparator. Inspired by dougcl's classic PLL patches (1, 2) and this post by Giftnudel.
PLLs seem to be a hot topic again. The jury is still out on whether they're used in the Buchla 259. Make Noise and WMD have announced PLL-equipped VCOs. They form part of the Wogglebug's chaotic heart and can be heard sputtering in the Hertz Donut's bad tracking modes.
I'd read about the inner workings of Doepfer's A-196 and PLLs in general in Horowitz/ Hill's Art of Electronics, but only just tried Giftnudel's patch. The A-196 employs a linear VCO, but as you can hear, the patch works just as well with standard, exponential VCOs:
minmax-pll (master left, slave right)
Two VCOs are compared in a min/max module like the Doepfer A-172. The minimum signal is subtracted from the maximum and the difference fed back to the 'slave' via a slew limiter, thus generating a CV. Depending on the initial frequency difference between the two VCOs and the lag time, you can imitate the wonderful glitches made famous by dougcl.
One nice thing about using this method over traditional sync is that it doesn't affect the sound of the slave as much. The other is that the slew limiter offers control over how well, or poorly, the oscillator tracks.