AllAboutBirds.org: learn your ornithology from Cornell
I am a very amateur birdwatcher, but I watch and notice birds every day: in my yard, on hikes and runs, out the car window. I have a fair number of guidebooks and can identify most of the common birds I see around me: pileated woodpeckers, downy woodpeckers, osprey, black capped chickadees, red-tailed hawks, barred owls, phoebes, juncos (some day I'll start a life list), but I'm not as good with some of the bird calls.
The other day I somehow ended up on the Cornell Lab of Ornithology's All About Birds website. The site offers an online bird guide with a lot of info (all for free) on many birds: keys to identification (size, shape, color, behavior, habitat), life history of the bird, audio of various songs, and video. Each bird's entry is neatly organized, and the information, along with the bird songs and videos, makes the site very worthwhile.
For more in-depth info, naturalists can subscribe to The Birds of North America ($42/year). However, for the casual amateur, the All About Birds site alone has a trove of information.
Here's a "cool fact" I learned about black capped chickadees: Every autumn Black-capped Chickadees allow brain neurons containing old information to die, replacing them with new neurons so they can adapt to changes in their social flocks and environment even with their tiny brains.
You even can get your bird fix from the office. The site links to several live nest cams. I watched a barn owl in a nesting box for several minutes, till it went to sleep. And Cornell's Macaulay Library has the largest archive of animal sounds and videos in the world. I searched for "pileated woodpecker" and got 412 audio and video results.
The All About Birds site is a great find. I just wish they had an iPhone app version, so I could take it with me. (I've been debating between some of the other current offerings.) In the meantime, I'll keep this site bookmarked.