Any book in the Marvin Redpost series. Great for first through fourth grade boys (and girls).
Sideways Stories from Wayside School. The above picture is a boxed set of the series. Hysterical. Even educational, and the best thing is, they read like a collection of short stories, so you can read a chapter aloud here and there to your kids. They will LOVE them.
Now I just realized this book is out of print, but Sixth Grade Secrets is probably one of my favorite middle grade books ever. Girls in particular will love it, but there are enough practical jokes in here for even a boy to love it. You can get it used for one cent right now. Get it at your library or buy it! Seriously, it is so good.
And of course:
Holes. One of the best middle grade novels of all time. Top ten, no question. Holes has a great sequel too. Small Steps.
And I hope everyone has read my absolute favorite Louis Sachar book ever:
There's a Boy in the Girl's Bathroom. Every adult and child on earth should read this book. And I don't say that lightly.
And finally, I really, really loved his latest book, which is geared toward Young Adult (all you mom's out there—this could be a book club book)—The Card Turner