Cornell Lab of Ornithology - All About Birds
http://www.birds.cornell.edu/AllAboutBirds/

Patuxent Wildlife Research Center - Bird Identification InfoCenter
http://www.mbr-pwrc.usgs.gov/id/framlst/infocenter.html

Birds of Oklahoma - Bill Horn, photographer - photos of many Oklahoma species
http://www.birdsofoklahoma.net

Pat's Backyard BirdCam - Pat Velte, photographer - photos and videos of many Oklahoma species
http://www.backyardbirdcam.com

Okie BirdCam - Terri Underhill, photographer - photos of many Oklahoma species
http://www.okiebirdcam.com

eNature.com - Online field guides
http://www.enature.com/fieldguides/intermediate.asp?curGroupID=1

Birding.com
http://www.birding.com/bird_identification.asp

Name That Bird - experts help identify birds
http://www.namethatbird.com/

Other resources

Oklahoma Ornithological Society
http://www.okbirds.org

George M. Sutton Avian Research Center - Bartlesville, OK
http://www.suttoncenter.org/

WildCare Foundation - Wildlife rehabilitation - Noble, OK
http://www.wildcareoklahoma.org

OKBirds Listserv
http://www.suttoncenter.org/okbirds.html to subscribe
http://www.birdingonthe.net/mailinglists/OKLA.html  to read online