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.