Phil Plait is my favourite astronomy blogger; this is an amazing top ten list.
Phil says “I went through hundreds of images (maybe thousands), checking NASA, APOD, the ESA, BAUT, and a few dozen amateur and professional sites featuring pictures as well. The criteria I kept in mind were beauty, of course, but also scientific value. But both of these could be trumped by the coolness factor.”
Robots on Mars
“This is a very small piece of a HiRISE image of Victoria crater on Mars, showing just one part of the rim. And sitting right there on the edge of the crater is a little metal robot named Opportunity. Launched in July 2003, that rover was designed to work for only 90 days, yet it just celebrated its 1000th day on Mars!”
The Shuttle, the ISS, and the Sun
“Not only did (Thierry Legault) get a picture of the Shuttle and the ISS, he nailed them while, from his viewing point, they were passing directly in front of the Sun.”
“This image has it all. It’s of a familiar object, seen in an unfamiliar way: back-lit by the Sun, a view impossible from Earth.” But, this image has a hidden surprise – a very distant image of the Earth!