2 of the best off-leash in the West Island:
Angell Woods in Beaconsfield, right near highway 20/Woodland and Elm. There is a huge open field and woods with tons of trails to walk through:
http://goo.gl/maps/CwIbl
In the summer you can also go near Baie d'Urfe yaught club, big open grassy area and dogs can go swimming:
http://goo.gl/maps/mCdCr