A recent article in Geology analyzes an evaporated lake on Mars, within the vicinity of the Opportunity rover. The researchers looked at a chloride deposit that was left behind when the lake evaporated. We see this same process occurring on Earth, such as at the Bonneville Salt Flats.
The geologists estimate that the lake held water about 3.6 billion years ago. This would have been one of the last lakes on Mars to hold water before the majority of the planet's water receded to its north and south poles.
The lake was slightly saline, being about 8% of the salinity of the Earth's oceans. Geologists feel that this former lake, and other similar chloride deposits may have represented some of the last habitable surfaces on Mars.