Here are just a few:
harrypotterandthehalfbloodprince.com, .net and .org (Warner Bros.)
harrypotterandthehalfbloodprince.info, .us, .co.uk (Seabottom Productions Ltd.)
halfbloodprince.com, .net, .co.uk (Lightmaker Group)
halfbloodprince.org (Warner Bros.)
Truth be told, HPANA snapped up halfbloodprince.info and we're planning an unofficial mini-site there with plenty of information on the upcoming book! If you're an older fan with some time on your hands and would like to help administer it, please contact us.
Warner Bros., of course, makes the movie versions of Harry Potter. Seabottom Productions Ltd. is related to Warner Bros. in the UK, and Lightmaker Group works on Rowling's official site.