Yahoo! Reversi is a very popular site. You can always find more than hundred people to play with. Its weaknesses include lack of security system, and therefore a lot of cheaters, and competitive rating system. Some people refuse to play with lower ranked players. But the ease of software installation is quite attractive.
MSN Zone is another popular site. The chat room is not as good as Yahoo!, therefore it seems a bit cold. The graphic interface, metallic board and non-ranking system are its attractions.
VOG may be the best environment, but comes at a price. You need to pay in order to enjoy its full benefits. There are many players most of the time. Bots (AI) are always available though.