Birding on the Web: The Next Generation - Where the birds are - to chase or just to dream; interesting chat on sites and birds; a great classification of world birds.
Birding on the Web - More great birding info.
Natural History Book Services - A great reference for new material - what's available & what it's about.
Wildflowers - Sources for books in the field as well as general information.
Weather - A worthwhile check before heading out into the field.