Updated November 3, 2011
All these links are live but I make no claims about the quantity or quality of their listings. Anyway, nothing beats walking the neighbourhoods you like, with notebook and possibly a camera, and looking for À LOUER signs.
housingmaps and padmapper both place apartment listings from craigslist and other sources onto Google maps
appartalouer.com
Campus Montreal
Concordia student union off-campus job and housing bank
chercheappart.com
craigslist
ifind.ca
Journal de Montréal classifieds
kijiji
La Presse classifieds
Lebonplancondo (lists rentals)
Les petites annonces classées lespac.com
locanto
louer.com
McGill housing classifieds
monbazar.ca
Montréal en ligne
More Montreal / Tout Montréal
The Gazette
Top Annonces
UQÀM listings
Voir classifieds with the option to pay to see ads in advance of publication