Where Can I Read Harry Potter And The Half Blooded Prince Online For Free?

I am not sure that you can.

However, you might try and look for low cost copies on such sites as Amazon.

It would definitely be worth looking on auction sites such as e-bay, second hand bookshops and thrift stores.  Although the film has only just come out, the book has been published a few years now.  People may be looking to make a few cents on spare copies etc by now.

Best of all, if you really want a free read rather than to own the book, why don't you ask round your family and friends and try to borrow a copy.  Failing that, you can definitely read it for free at your local library, providing you don't keep it so long you start incurring fines.  If they do not have it on their shelves, the librarian will always order a copy for you to read if you ask.