Ummm, I think alot of these things have been said, but I'm not too sure. So I'll put them just in case.
I found them in a New Zealand magazine.
1. That cose thing Mr Weasley dialled to get onto the Ministry of Magic was 62442, That spells magic.
2. Harry's middle name is James (Duh), Ginny's is Molly, Hermione's is Jane and Ron's is Bilius.
3. J.K named the Knight Bus staff after her Grandad's, Ernie and Stanley.
4. A magical quill detects the birth of every magical child and then Professor McGonagall sends an owl to each child when they turn 11.
5. Harry is frequently told he has his mum's eyes. They'll be vital to the plot.
6. When DD heard LV had used Harry's blood, the book said there was a gleam of triumph in his eyes, meaning LV had sealed his own fate.
7. Usually wizard children are homw-schooled before going to Hogwarts, because they can't be trusted to keep their magical abilities hidden from muggle children.
8. The core of Hermione's wand is dragon heartstring, so Harry Ron and Hermione unite the three Ollivanders wand cores.
9. Natalie MacDonald, who apperas on page 159 of Harry Potter and the GoF, is a real person. She was a nine year old girl from Canada, who was dying of cancer. She wrote to J.K. asking what happened in the next HP book as she wouldn't live long enough to read it. J.K. emailed back, but Natalie had died a day earlier. In tribute, she became a first year student at Hogwarts and was put in Gryffindor.
10. Members of the OotP communicate using their patronuses.
11. Crookshanks is half Kneazle.
12. If J.K. writes an eighth book, it'll be an encyclopaedia of the wizarding worldusing all the back-groundon characters left out if the books.
This was just some useless trivia, and I wasn't sure where to put it, so I stuck it here because it may have spoilers or something.