Professional Organizations
Examples of other bird club web sites
Identifying birds by sight and sound
Web sites with many birding topics
Other birding information of various types

Professional Organizations:
Pennsylvania Society for Ornithology
National Audubon Society
Cornell Lab of Ornithology
American Birding Association
American Ornitholgist's Union
Birdnet (Ornithological Information)
Selected bird club web sites:
Delaware Vally Ornithological Club
Seneca Rocks Audubon Society
Lehigh Valley Audubon Society
Presque Isle Audubon Society
New Jersey Audubon Society
Howard County (MD)
Northern Virginia Bird Club
Cincinnati Bird Club (OH)
Baltimore Bird Club
Southern Maryland Audubon Society
An extensive list of PA birding organizations
Worldwide Directory of Online Bird Clubs
Identifying birds by sight and sound:

Cornell University Bird Guide
Cornell Macaulay Sound & Video Catalog
whatbird.com's complete online Field Guide with pictures, songs, range maps, etc
Feather Quest: identify birds by their feathers
National Geographic field guide in a handheld PDA
Patuxent Bird Id InfoCenter
Patuxent Wildlife Research Center songlist

Web sites about the general topic of birding:

birding.com's list of it's top 25 Birding Websites
birding.about.com
Birdwatching.com
birdingonthe.net
Birdzilla.com
Birder.com

Other birding information of various types:

Birding contacts around the world
PA, Centre County, and Townships maps
Accessories: bigpockets.com
Birdsource: national and state Birding Projects
Fatbirder: linking birders worldwide
Central PA Birdline
Checklists for all states and selected parts of the world
Virtualbirder.com: Take a virtual online tour of birding locations
Bird Watcher's Digest
Peterson Online
Hawk Mountain Sanctuary
Cape May Bird Observatory
Cape May Birds and Birding
Cape May Hawk Watch
Pennsylvania Game Commission
Mapquest (maps and trip directions)
Google maps (maps and trip directions)