Non-equilibrium phase transitions may mean different things, depending on the context: it could refer to dynamical critical phenomena or to dissipative structures.
Dynamic critical phenomena are usual critical phenomena (aka equilibrium phase transitions) studied under non-equilibrium conditions. For example, one may consider behavior of an Ising farromagnet when a temperature is swept across critical point at a finite speed, or when the change of magnétisation is caused by varying magnetic field. One could also study response of a strongly correlated system beyond the linear response, as, e.g., non-equilibrium transport through superconducting islands.
Dissipative structures are symmetry breaking phenomena occurring in systems out of equilibrium. Unlike in the case of the dynamical critical phenomena, different phases may not exist when the system is in equilibrium, but appear only when there are non-zero fluxes of energy, matter, etc. flowing through the system. A classical example is Benard convection in a heated liquid. More important applications are explaining the origin and existence of life or the Earth's geochemistry.
Theory of the dissipative structures, and indeed non-equilibrium thermodynamics, was pioneered and developed by Prigogine and his school. It however remains controversial - partially because some if its ideas are poorly understood/popularized by the broader community (e.g., the coexistence of minimum entropy production principle and maximum entropy production principle, see this post), but also because its applicability to key problems, such as the origin of life on Earth, has not been convincingly demonstrated - see the Anderson's critique.
Moreover, although studying of symmetry breaking in non-equilibrium conditions is certainly a part of physics, the problems of interest are often of interdisciplinary nature, and typically tackled by people with no special knowledge in in critical phenomena or even thermodynamics, and published in journals overlooked by the majority of physicists (e.g., see here). On the other hand, the examples accessible to physicist of general background, such as Benard convection or Schlögl model, are too specific to be of broad interest and stimulate extensive theoretical efforts.