GEO600 was the first gravitational-wave detector to use beam centering of the wavefront sensors and the first to describe the particular alignment geometry of three-mirror mode cleaners.
Beam centering of wavefront sensors is used by Advanced LIGO as of today, and was a crucial step for stability when implemented in 2014. Virgo also uses wavefront sensors beam centering.
The LIGO Hanford instrument experimented with use of dark port signals for beam centering of the Michelson alignment to increase stability, a technique invented and first demonstrated by GEO600 to make the readout method more stable.
In all, 260 control loops are needed to move GEO600 mirrors into alignment, hold it there and dampen external vibrations on the system.