Wow. What an amazing list, so far. I had to run over to my bookshelf and see what else I could find (I have never thrown away a book, that I can remember, and my wife is an elementary school reading teacher, so we have a pretty massive library).
I may have missed it, but I haven't seen T.H White's
The Once and Future King yet. I remember liking that one, though I don't quite remember how old I was when I read it.
Someone mentioned Diana Wynne Jones' Chrestomancy series. That's another good one, especially well-suited as a follow-up for the kid who's just read all the Harry Potter books, and doesn't know what to do next.
Phillip Pullman's Dark Materials trilogy is phenomenal. Absolutely, breathtakingly phenomenal, and full of brialliant and comlplex ideas. If anyone on this list hasn't read it yet, you should, regardless of age. One thing to be aware of, though, it is DARK. Some of the things that happen to the kids in these books are downright tragic. It could be upsetting, so use your judgement if your kids are ready for it.
A relatively new author of kids/young adult fantasy that I like a lot is Garth Nix. He's got two books out right now that I'd recommend: Sabriel and Lirael. A third is on the way sometime later this year I think. The series is about a girl who is the last in a long line of hereditary necromancers, who use their powers to put the dead to rest, rather than raise them. Very well written, and much more accessible than it sounds from my description. Definitely worth checking out.
<edit> here's a link to the publisher's homepage:
http://www.garthnix.co.uk
-F5