“Salinity energy” stored as the salinity difference between seawater and freshwater is a large-scale renewable resource that can be harvested and converted to electricity, but extracting it efficiently as a form of useful energy remains a challenge. With the development of membrane science and technology, membrane-based techniques for energy extraction from water salinity, such as pressure-retarded osmosis and reverse electro-dialysis, have seen tremendous development in recent years. Meanwhile, many other novel methods for harvesting exergy from water mixing processes, such as electrochemical capacitor and nano-fluidic energy harvesting systems, have been proposed. In this work, an overview and state-of-the-art of the current technologies for sustainable power generation from the water salinity gradient are presented. Characteristics of these technologies are analyzed and compared for this particular application. Based on these entropic energy extracting methods, the water salinity, as the “blue energy”, will be another source of renewable energy to satisfy the ever-growing energy demand of human society.