A very close shave: on October 18/19, comet Siding Spring (C/2013 A1, discovered by our very own Rob McNaught) will pass Mars at a minuscule distance of ~130,000 km.
Mars & comet should be readily visible together in a single field of view.
http://mars.jpl.nasa.gov/comets/sidingspring/
The approach is so close that NASA/JPL/India are arranging for their Mars-orbiting spacecraft to be on the other side of the planet so as to gain some refuge from particulate damage!
Dean