JK Rowling's own site(the truely only trustable source of info) http://www.jkrowling.com/
Sites JK has handed awards out to http://www.hp-lexicon.org/
This site is loaded with info. They figured out the entire time line and have family trees and much much more. Make sure you have a fw hours to burn.
Warner Brothers HP site http://harrypotter.warnerbros.com/main/homepage/home.html
Movie info, release dates etc...
schoolastic HP site http://www.scholastic.com/harrypotter/home.asp
The american publishing company for Jk Rowling.