The Web Accessibility Center at the Ohio State University has some excellent resources and examples. After you've explored there, you can spend many happy hours wandering the resources in their page o' links.

The single most useful site is WebAIM -- Web Accessibility In Mind. There you'll find robust discussions of accessibility standards in development, tutorials, standards, and answers to many questions: soup to nuts, experts to beginners.

For example, here's their survey of how 665 daily screen reader users experience the web.