NASA has just released a statement saying that they've got good evidence of liquid water making an appearence on Mars some time in the last seven years.
BBC News Story
This is very exciting for several reasons. There is water on Mars that we know of, but it's all locked in ice. (As an aside, did you realize that ice is a mineral? It's the lowest density mineral that occurs on Earth.) But the most likely cause of this new gully that you can see in the photograph is liquid water! Liquid water is important because it's necessary for life as we know it (so it once again introduces the possibility that we may some day find some sort of single-celled life on the red planet) and it's also very important if we ever want to consider the possibility of building some sort of base or research station on Mars. We need a lot of water to survive, and it would make getting there and setting up shop a lot easier if we didn't have to haul all of the water necessary with us.
Now, there has been evidence suggesting liquid water, though all of that was for much less recent events. A lot of the erosional forms on the surface of the planet point toward liquid water, though it could also be argued that the erosion could be caused by wind as well. (If you're not actually there to look at things, it can be very difficult to tell the difference between erosion caused by liquid, wind, and simple wasting of loose material.) And there still is a possibility that this new gully was caused by mass wasting or even liquid carbon dioxide.
But liquid water is currently the best explanation.
This is all very exciting stuff!
High resolution images from the Mars Global Surveyor.
Phil Plait comments.