We present a compact, highly portable and inexpensive interferometric module for obtaining spatial interferograms of microscopic biological samples, without the strict stability and the highly-coherent illumination that are usually required for interferometric microscopy setups. The module is built using off-the-shelf optical elements and can easily operate with low-coherence illumination, while being positioned in the output of a conventional transmission microscope. The interferograms are processed into the quantitative amplitude and phase profiles of the sample. Based on the phase profile, the optical thickness or optical path delay profile of the sample is obtained with temporal and spatial stabilities, at the order of 0.2-0.3 nm. We show several configurations of this interferometer that are suitable for both on-axis and off-axis holographic geometries, and present various experimental results, including imaging live cells in a non-contact label-free manner and transparent elements with nano-scale thickness. Since the interferometer can be connected to the output of a transmission microscope and operate in a simple way, without involvement of an expert user with a knowledge in optics and without complicated alignment prior to every experiment, and still obtain a remarkably high accuracy, we believe that this new setup will make interferometric phase microscopy more accessible and affordable for biologists and clinicians, while significantly broadening its range of applications.