Brian,

You may want to start with Media Metrix. They have some Top lists by unique visitors:

http://www.mediametrix.com/TopRankings/TopRankings.html

http://www.mediametrix.com/PressRoom.../03_21_00.html

I have a few more somewhere around here. I'll see if I can find them and will post them.

Good luck.

Mark

http://www.siteworkz.com