We describe a digital lock-in amplifier instrument which is designed to optimally extract the amplitude and phase of response at intermodulation product frequencies. This general purpose laboratory instrument has broad application in the analysis of nonlinear systems. A defining property of the intermodulation lock-in is, that the Fourier components of the drive waveform and the sampling frequency used to sample the response are all integer multiples, or close to integer multiples, of a fundamental base frequency Δf. We describe an implementation of the intermodulation lock-in with a field programmable gate array. Applications of the intermodulation lock-in are discussed, including its use in Atomic Force Microscopy.