Young adult books can be a slippery slope, as they can be aimed at anywhere from 12 to 18 year olds-and obviously, topics appropriate for an 18 year old might be way too mature for a 12 year old. Some are written to deal with current, pertinent topics-and for an 18 year old, that might be date rape, substance abuse, etc. Award winners are often some of the most topical-not always what you might want a 13 year old who is, as you say, an "average all-around kid" to be reading.
There are many awards for juvenile literature-I have attached a link to one good library site.

Here is a link that might be useful: juvenile literature awards

I have a 10 year old girl and 13 year old boy. My son is not a big fiction reader, he reads for information, but he loves Artemis Fowl or anything by Eoin Colfer. He also loves Calvin & Hobbes, Foxtrot and Tintin comics.

I think that consulting with a good children's librarian may be better than the awards (though the Newberry is always pretty reliable), because some of the YA award books can have some material you might consider objectionable. For example the book Tender Morsels that had a thread on here not long ago.

Here's a list of books my kids have really enjoyed:

Mrs. Frisby and the Rats of NIMH
The Tale of Desperaux
Any of the Magic Treehouse books
The Amazing Maurice and his Educated Rodents
Andrew Clements' books (Frindle, Janitor's Boy etc...)
Dragon Rider

Here's what's on the eighth grade summer reader list at my son's school:

They all have to read Warriors Don't Cry by Melba Patilla Beals, then they can choose any of the following:
I Know Why the Caged Bird Sings - Maya Angelou
It's Not About the Bike - Lance Armstrong
Tasting the Sky: A Palestinian Childhood - Ibtisam Barakat
A Walk in the Woods - Bill Bryson
Facing the Lion Growing up Maasai on the African Savanna - Joseph Lemasolai Lekuton
In the Heart of the Sea: the Tragedy of the Whaleship Essex - Nathaniel Philbrick (we live near Boston, so local history)
Chew on This - Eric Schlosser
The Man Who Ate the 747 - Ben Sherwood
Big Mouth and Ugly Girl - Joyce Carol Oates
Travel Team - Mike Lupica
A Step From Heaven - Na, An
A Hitchhiker's Guide to the Galaxy - Douglas Adams
Summerland - Michael Chabon
Dune - Frank Herbert
The Green Mile- Steven King

There are a few others on the list.

For the younger ones, there are some really great picture books:

The Olivia books by Ian Falconer
The Three Pigs by David Wiesner
The Miss Spider books
Jan Brett books are a feast for the eyes

Hope that helps.

Check out this link to the YALSA site for lists of award-winning YA books. I completely agree with CC, though, in that YA encompasses 6th through high school or beyond, and some of what is offered for the upper age range isn't what most folks want a 13 yr old reading.

Check out the Maryland's Black-Eyed Susan award. The books are broken down by age groups; the nominees are chosen by mostly school librarians and school students read the books and vote for the winners. You'll find wonderful choices for both boys and girls on these lists.
http://www.maslmd.org/awards_winners.php

Here is a link that might be useful: award winning YA books

Sorry, here is a clickable link to the Black-Eyed Susan nominees and winners for this year.

Here is a link that might be useful: Black-Eyed Susans

These are wonderful suggestions - thanks so much.

I've also bought The Read Aloud Handbook as I know that will help with choices as well. Many of you have recommended this over the years....

My 14 yo and 10 yo both enjoyed Rick Riordan's Percy Jackson and the Olympians series and Erin Hunter's Warriors series. My DD was a big mystery reader through 3rd and 4th grade and liked the A to Z Mysteries, the old Dana Girls series, Jigsaw Jones and The Magic Treehouse, too.

When they were younger they both liked Mrs. Piggle-Wiggle and The Boxcar Children (but only up to book 18 or so -- the rest were written by someone else and aren't nearly as good as the first group).

My 14 yo also loved the Twilight books. And, of course, Harry Potter. Last year she was into the Cornelia Funke books, Inkspell, Inkdeath, and a third whose name escapes me at the moment. Another series she loved was Terry Pratchett's, The Wee Free Men, A Hat Full of Sky and Wintersmith.

There are a ton of great YA books out there, but as others have warned, some have very (IMO) dark and adult themes to them.

We have a ton of YA books in the house, I'll take another look and post some more later. Hope this helps!

Some recs for gift books off the top of my head, based on kid's bookshelves:

picture books for the youngest ones:
King Bidgood's in the Bathtub by Wood
Sandra Boynton books (specially the Philadelphia chickens song book)
Toot & Puddle books by Holly Hobbie

and practically everything sheriz mentioned

Just want to add that my 12yo would also rec Eva Ibbotson (also has kids books), Tamora Pierce (YA), Diana Wynne Jones, Jane Yolen ...

@sheriz -- did your 14yo read the last Twilight book -- what did she think? I haven't let my 12yo read it though she's read the others

Ajpa, my DD read the last Twilight last summer and, aside from saying the birth scene was disgusting, she really loved the book. She likes scary books though (she spent 4th grade devouring the Goosebumps books). I have not read them myself so I can't comment beyond that. She's re-read the series several times and has now moved on to another teen vampire series.

Lemonhead, there was another series I meant to recommend, too: Jeanne DuPrau's City of Ember, People of Sparks, Diamond of Darkhold and The Prophet of Yonwood. Both kids read those last summer and enjoyed them.

From the 10 yo DS's shelf: The Boy Who Saved Baseball by John H. Ritter, MVP by Douglas Evans and John Shelley, The Name of this Book is Secret! and If You're Reading This It's Too Late! by Pseudonymous Bosch, and The Series of Unfortunate Events, by Lemony Snicket. And finally, in the sort of graphic novel category, the three Diary of a Wimpy Kid books by Jeff Kinney.

Last but not least, my DD really liked the Uglies series by Scott Westerfeld. She read them when she was 11 or 12 -- Uglies, Pretties, Extras and Specials.