Was flicking around on Gamasutra for some interesting articles, and came across this 2-part Feature by Pascal Luban, who writes a lot about Online Games.
As the title says, it talks about the overall design of Free-To-Play (or F2P) onlines games, that a lot of studios are now beginning to implement into their games. Some famous examples include Battlefield 2, Star Trek Online, and Lord of the Rings Online.
It's a very interesting article, and I highly recommend reading it!
Part 1 - http://www.gamasutra.com/view/feature/6552/the_design_of_freetoplay_games_.php
Part 2 - http://www.gamasutra.com/view/feature/6592/the_design_of_freetoplay_games_.php
Enjoy!