Comet ISON will pass Mars at around 1600 GMT (noon in the eastern USA) today at a distance of about 7 million miles. Unfortunately, it is right beside the moon this morning in Leo, so it would be quite difficult to see even with a telescope. (The moon will be out-of-the-way after Oct 5).
I made a video and a Mathematica demonstration showing the path of ISON through the solar system, but there is a much nicer interactive viewer at solarsystemscope.com.