Being a DIY kind of guy I had to make my own Apple Boxes. :-)
A full set of both full and mini Apple boxes for under $45.
Another $15 for the case.
It took an afternoon to build them.

Bill

The boxes and the case
http://cinebydesign.net/images/apple-2.jpg

How to make them.
http://cinebydesign.net/apple-boxes.htm

What are apple boxes? They just look like plywood boxes + black paint?

Why an apple box is the brand new way to capture uncompressed HD right on set!!

Just kidding, but I wish. Its surprising how useful grip equipment can be so simple.

An apple box is exactly what you stated : a pine box. However they come in various sizes and are used a number of ways.

For instance, I have used apple boxes as...

a make shift high hat
a platform for a high angle shot
a platform for lights
a stool
a platform to cheat furniture, or other items up in a shot

Yeah - basically I'm all platformed out.

Anybody else care to share their wild and crazy uses for an apple box(es)

As a riser/support for a camera in a car (with the camera properly strapped down of course).

My favorite technique for building an Apple-Box is: Dump miscellaneous electrical equipment out of the milk crate you're carrying it in, turn over the milk crate, finished.

Patrick, an apple box typically refers to something you stand on or set something on to get it higher. Like a short person trying to see into a higher placed camera viewfinder would stand on an apple box to get up to it. It comes from the traditional practice of actually using an "apple" box, the kind apples used to come in, but now it generically refers to pretty much any kind of handy box used on set, since apples now come in bags and when you stand on those you don't get much higher. (hehe)