Forum Moderators: open
We might ask "How many active top level domains are there that serve unique content?" -- and then we still run into the question of how to detect duplicate content.
In all honesty, I only browse about 15 or so on a regular basis:
nytimes.com
webmasterworld.com
an actuate forum
a band director forum
another band director forum
cnn.com
pcpitstop.com
several of my own sites
frontpage magazine
google
yahoo
plus an occasional one I find for my kids school work and an occasional shopping jaunt to lands end.
But that's about it. So, for the beelions and beelions of available sites, I just have my few.