Party Poker (Popularity: )
http://www.partypoker.com/ Play for real money with players from around the world. Hosts PartyPoker Million tournament each year.
Poker Pulse (Popularity: )
http://www.pokerpulse.com/ Hourly tracking and daily summaries of player numbers at many of the most popular online cardrooms.
True Poker (Popularity: )
http://www.truepoker.com/ 3-D Texas Holdem and Omaha ring games and multi-table tournaments in both free and real money environments.