The NASA press conference today confirmed what many of the rumours were saying, NASA has found strong evidence for the very recent presence of liquid water on the surface of Mars. Mars Global Surveyor images have shown the formation of gullies in places where there were clearly not just a few years ago. Perhaps the most likely explanation for the formation of the gullies is liquid water.
There's plenty of images on the Malin Space Science Systems website. Also check out the BBC News story.
The MGS team have also released some excellent images of recent impact craters on Mars.
UPDATE: The Planetary Society website has an excellent article about this news story, and further comment on Emily Lakdawalla's blog.