Well itaal has pretty much covered the close-by areas, since McGill is right in the downtown there's not much in the way of housing within a 5-minute walk. If you're willing to metro in, you can look for places on the Green line of the metro. A really useful tool for visualizing distances from campus is padmapper: http://www.padmapper.com/
And McGill's off-campus housing website has some useful resources, e.g. brief descriptions of each neighbourhood: http://www.mcgill.ca/offcampus/start/location/
Depending on what your priorities are (e.g. apartment size, safety of neighbourhood, ease of commute, price), I can give you a better idea of what neighbourhoods you might want to be looking in. The area around Lionel-Groulx metro (St-Henri) has some quite big places for good prices if you don't mind about a 10-minute metro commute (3-4 metro stops).