According to Fiction Matters, the Big Six Publishers are:
- Hachette Book Group
- MacMillan Publishers Ltd
- Penguin Group
- Random House
- Simon & Schuster
I know I live under a rock (what can I say, it's cozy), but I've never heard of Hachette Book Group (though I know some of their imprints). All the others are familiar. At least a little.
For your edification, Fiction Matters spells them out in detail at Fiction Matters' "Who Are The Big Six?"
Disclaimer: the article I got this info from was first posted in March 2010.
picture credited to http://ddeubel.edublogs.org/?s=competition