An inexpensive non-contact sensor for measuring the velocity of a broad class of targets, including liquid suspensions and diffusely scattering surfaces, has been developed and field tested. The operating principle is based on (1) illuminating the point of observation with two coherent beams, incident at different angles, (2) detecting light from both beams which has been scattered in a common direction, and (3) observing the interference between these two scattered beams as a function of time. When the target moves, the phase of the interference signal changes at a rate which is proportional to its velocity, thus allowing the direct measurement of the instantaneous surface or flow velocity. In the past, Laser Doppler Velocimetry (LDV) techniques have been largely limited to situations where single small particles pass through the illuminated region, generally giving rise to clean signals with high modulation efficiency. In this paper we describe an extension of this technique to the measurement of aggregates of particles in liquid suspension, or even diffusely scattering solid surfaces. In these cases, the interference signals are weak and are superposed on large background levels which fluctuate randomly with time. Accurate detection of these signals requires appropriate optical geometries and data processing techniques. We describe the design of such a sensor, and present test results. A measurement stability (precision) of better than 0.1% has been achieved in tests with paper pulp, and the indicated velocity correlates very well with the data from a bulk flow sensor. We use a laser diode source, PIN-diode detectors, inexpensive optical components, and an efficient digital data processing system: this leads to a sensor which is both compact and low in cost.