When I saw this photo, the little hairs on the back of my neck stood up.
It is a picture of the Phoenix lander, as it parachutes down to the Martian surface. The lander is the smaller of the 2 bright spots; you can just make out the parachute shrouds connecting it to the canopy above. This picture was taken from the Mars Reconnaissance Orbiter's HiRISE camera, which left Earth in August of 2005 and has been in orbit around Mars for the last year or so.
Wow. This is equivalent to having a picture taken of a speeding bullet BY a speeding bullet. Another analogy I heard was that the Phoenix mission is like sinking a hole-in-one by teeing off in Washington DC and the cup being in Australia. This picture is akin to having a pre-programmed camera present to take a picture of the ball just before it goes in.
The Bad Astronomer over at the BadAstronomy blog(nice to see a non-political post for a change) summed it up the best:
Think on this, and think on it carefully: you are seeing a manmade object falling gracefully and with intent to the surface of an alien world, as seen by another manmade object already circling that world, both of them acting robotically, and both of them hundreds of million of kilometers away.
Oh yeah. Coooool.