We are uncovering significant optical variability in low/hard state observations of several X-ray binaries on the fastest time-scales of just tens of milliseconds typically probed with modern rapid imaging cameras. The optical light curves are remarkable in that they display properties very characteristic of X-ray variations: 1) power spectra with band-limited, red noise over broad time ranges of 10 ms - 1000 s, and in some cases, a low-frequency quasi-periodic oscillation; 2) an instantaneous variability amplitude linearly scaling with source flux; and, 3) log-normal distributions of fluxes. Aperiodic optical variability components can dominate over simple linear X-ray reprocessing expectations, and are much faster than viscous time-scales of the outer accretion disk or flow. Cross-correlated optical vs. X-ray time delays not only constrain emission mechanisms, but can also be used to probe characteristic size scales of the physical components (jet, corona), and to understand how they are coupled. Rapid, multiwavelength timing studies are thus opening a new window on the hearts of accreting sources, though the broad-band spectral plus timing properties remain to be unified consistently. I will briefly review recent results on rapid optical variability, including our new data on black hole and neutron star binary systems. The fact that the sources were all in typical low/hard states (with relatively-bright optical counterparts) suggests that correlated optical/X-ray activity may be a general feature, waiting to be uncovered in more systems. The continuance of RXTE is vital for such work.