Great places to browse and buy books:

Amazon
Booksense
Charlesbridge Publishing
Clarion Books
The Children’s Book Council
Farrar, Straus & Giroux Books for Young Readers
The Horn Book
Junior Library Guild

Great places for writers and aspiring writers:

Agent Query
Children’s Writer’s and Illustrator’s Chat Board
Institute of Children’s Literature
Jacket Flap – The Children’s Book Resource
The Author’s Guild
The Purple Crayon
Society of Children’s Book Writers and Illustrators