One of the birding sites I visit most frequently is The Cornell Lab of Ornithology website called All About Birds. It is a great looking website and the incredible content is very intuitive to navigate. Every birder should have this website link saved in their web browser favorites!
I use their online Bird Guide the most. You will notice in my blog posts, whenever a bird name is mentioned I insert a hyperlink to that bird's profile on All About Birds Bird Guide whenever possible. They don't have every North American bird on their yet, but they've got most of them and they are working on adding more. I really enjoy the searchability of this guide. As you start to type in the name of the bird it automatically pulls up a list of birds with those letters in it to help you quickly narrow it down to the bird you are looking for.
The content for each bird is really nice. There are four tabs for each bird: Identification, Life History, Sound, and Video. I click on the first two tabs all the time with the Cool Facts under Life History being my favorite section of all. I love maps and the range maps with an additional link to eBird's maps are very handy.
The online Bird Guide is fantastic, but that is not all you can get from this website. I have also used the Birding Basics feature to brush up on my own techniques but also to learn how to teach birding skills to others on the bird walks I lead. The text, artwork, diagrams, and videos are all of top notch quality and are extremely helpful.
The Living Bird section is loaded with great stuff and you can even read much of Living Bird Magazine online here. I use this website daily, but I haven't even begun to scratch the service of all that the site offers. Check it out for yourself!