Many of these organizations have an abundance of information and further links to other useful and interesting pages.
Ontario Stewardship http://www.ontariostewardship.org/
Oak Ridges Moraine Land Trust http://www.oakridgesmoraine.org/
Oak Ridges Moraine Foundation http://www.ormf.com
Oak Ridges Trail Association http://www.orta.on.ca/
Stewardship Canada http://www.stewardshipcanada.ca/sc_national/main/index.asp?sProv=ca
Federation of Ontario Naturalists http://www.ontarionature.org/index.php3
Conservation Ontario and links to every conservation authority http://conservation-ontario.on.ca/
Ducks Unlimited http://www.ducks.ca/
Evergreen http://www.evergreen.ca/
Ontario Heritage Foundation http://www.heritagefdn.on.ca/
Youth Programs of the MNR http://www.mnr.gov.on.ca/MNR/youthprograms/steward.html
Ontario Federation of Anglers and Hunters http://www.ofah.org/
Ontario Woodlot Association http://www.ont-woodlot-assoc.org/
The Wetland Habitat Fund http://www.wetlandfund.com/
Nature Conservancy of Canada http://www.natureconservancy.ca/files/index.asp
The Centre of Land and Water Stewardship, University of Guelph http://www.uoguelph.ca/~claws/
The Ontario Environmental Directory http://www.oen.ca/dir/