Geologist and amateur space image processor Justin Cowart has been taking a dive into Mars Express' High Resolution Stereo Camera (HRSC) data lately and posting some of the results on his Flickr page. The best science from HRSC has been done on their highest-resolution pictures, taken close to the planet, but I really enjoy the wider views that HRSC takes when much farther away from Mars on Mars Express' highly ellptical orbit.
I'm not sure I can recall a prettier picture of the four great volcanoes of Mars' Tharsis region:
And here's another broad view across the other major volcanic province of Mars, Elysium:
Here's Mars' north pole, a nice pair to the one that the mission released of the south pole a while back:
And finally, one higher-resolution image, containing a beautiful rime of morning frost in the mountains next to Hellas basin.