The Hubble has taken a number of images of Mars, but four sets of pictures are particularly notable. There are two opportunities every year when Mars and Earth are the closest they will be to each other. These biennial close approaches of Mars and Earth are identical; they vary every year, because the orbit of Mars around the Sun is extremely elliptical; the close approaches to Earth can range from 35 million to 63 million miles. When Mars and Earth are directly aligned, at their closest, they are said to be in "opposition." There's a super explanation here, even though the most recent example is from 2001.

The first, and in some ways, still the most stunning, was taken in June of 2001. Also be sure to take a look at this detailed image with call-outs labeling the more interesting features.

The second set of images was taken during Mars' closest approach to Earth in almost 60,000 years, in 2003. You'll notice that there are two images; each represents a different side of Mars, with the red planet making almost half a rotation between the first and second images.

The third image is from October, 2005. It's a day short of Mar's closest approach to Earth that year. There's a large dust storm visible in the image, shown as a somewhat brighter, darker red in the middle. The storm is 930 miles wide, and at the time of the image, had been raging for several weeks. At the time, it was almost summer on Mars, so the south polar ice cap is almost completely disappeared.

The last image is from December 2007, a day short of Mar's closest approach to earth for the year; it's the image linked to in this post. There aren't any major storms visible, but there are clouds at both poles.

If you haven't time to look at anything else, take a peek at this animation of Mars rotating, based on Hubble images from 2007, and this video of Hubble images showing Mars' in opposition to Earth, an event that happens every 26 months, every time it happened, from 1995 to 2007.