http://www.universalorlando.com/harrypotter/
This is a link to the official Harry Potter themepark website. The theme park is located Orlando Florida.
http://www.pottermore.com/
This is J.K. Rowling's official Harry Potter site. It is free and easy to use. In this website you can become a student at Hogwarts and explore the grounds.
http://www.mugglenet.com/
This is a totally awesome fanmade Harry Potter website. It's totally awesome for pictures and videos
http://www.the-leaky-cauldron.org/
Another Harry Potter Website of Awesomeness.