Recently, motivated both by basic scientific interest and technological applications, and utilizing both experiment and theory, a number of investigators have independently identified a small group of off-stoichiometric Heusler alloys as having unusually interesting magnetic properties. These alloys take the form Ni50−xCoxMn25+yZ25−y (Z = Sn, In, Ga, etc.), being Co-doped off-stoichiometric versions of the better-known Ni2MnZ full Heusler compounds. In certain critical composition ranges these alloys are found to display unusually reversible martensitic phase transformations, multiferroicity (due to coexisting ferroelasticity and magnetic order), heightened sensitivity to compositional changes, and acute magnetic phase competition, leading to such exotic phenomena as spontaneous nanoscale magnetic inhomogeneity, collective cluster freezing, and intrinsic exchange bias. In terms of applications they can exhibitmagnetic-field-induced phase transformations,magnetic shape memory behavior, magnetocaloric effects, and remarkably low thermal hysteresis, making them attractive for sensors and actuators, magnetic refrigeration, and energy conversion devices. In this chapter we briefly review the current state of knowledge on the magnetic properties of these alloys, before presenting new results on the prototypical Ni50−xCoxMn40Sn10 system in the critical composition range0 ≤ x ≤ 14. Combining comprehensive magnetometry, exchange bias studies, and both new and previously published neutron scattering data, we present a detailed picture of the magnetic phenomenology in this alloy system and construct a magnetic phase diagram. Most importantly, based on these results and the work of others, we discuss in detail potential origins of the unusual magnetic properties of these materials, most notably the magnetic phase competition and nanoscale inhomogeneity that dominate their low temperature magnetism.