Lauren Ipsum is kind of like the Phantom Tollbooth of computer science. I highly recommend it! It's being sold two ways, through No Starch Press or through Amazon and Kindle.

My six year old loved it, but older ages would as well.

Ds7 just read George's secret key to the universe by Stephen and Lucy Hawking. I was a long overdue step up for him and he enjoyed it.

Oh, and the Nick and Tesla books!

Watership Down.

Tailchaser's Song.

I love both of those books. My son is younger, so I'm not sure what level a gifted kid is reading at at this age range, but if Harry Potter is on the list, these two seem about that level. If nothing else, as read alouds.

Max Axiom "comic" science books. Non-fiction.

Watership Down is very sad and dark in places...not that HP isn't, but most people are aware of that. I'm about to give it to my 11yo, though she could have handled it younger, but not at 6.

Two favorites from my childhood that I've just rediscovered for DD7: While Mrs. Coverlet was Away, and Mrs. Coverlet's Magicians, both by Mary Nash.

Along the lines of the James Patterson suggestion:

"The Adventures of Vin Fiz" by Clive Cussler

DD8 just finished Look Into My Eyes, book 1 in the Ruby Redfort series (written by Lauren Child) and declared it one of her faves.

Age 6-8 covers a lot of territory... I'll make a couple of suggestions for the younger end of that range, mostly books at the normal grade 3/4/5 reading level, but which are interesting for young strong readers:

Fiction:
Phineas L MacGuire series, Frances O'Roark Dowell
Dragonbreath series, Ursula Vernon
Alec Flint mysteries, Jill Santopolo
The World According to Humphrey series, Betty G. Birney
The Year of the Dog (& sequels), Grace Lin
The Secret Science Alliance, Eleanor Davis
Gooney Bird Greene series, Lois Lowry
My Mom the Pirate, Jackie French
We Can't All Be Rattlesnakes, Patrick Jennings

Non-fiction(ish):
Did Fleming Rescue Churchill?, James Cross Giblin & Erik Brooks
Tell Me a Picture, Quentin Blake

Non-fiction:
Kids Discover magazine (www.kidsdiscover.com)
The Day-Glo Brothers, Chris Barton & Tony Persiani

For readers unfazed by length:

Fiction:
Magyk (& the rest of the Septimus Heap series), Angie Sage -- long fantasy books that are fun and the author has said she intended them to be suitable for age 8 (so they're lower on violence, etc., than, for instance, Harry Potter)