Nilton Renno's droplets that form where salt touches ice - why did he call a droplet of salty water on Mars "a swimming pool for a bacteria"?
Recurring Slope Lineae
Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over
- that one was reported as uninhabitable but Nilton Renno was not so sure, biofilms might make it habitable
Transgressing sand dunes bioreactor
Desert varnish
Sun warmed dust grains embedded in ice
Southern hemisphere flow-like features - these may involve fresh water!
Methane plumes on Mars and the possibility of water deep below the surface in its hydrosphere
Porous basalt
Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most)
Ice covered lakes habitable for thousands of years after large impacts
Ice covered lakes from volcanic activity
Possibility of geological hot spots in present day Mars
Life in ice towers hiding volcanic vents