According to a Develop source, the famous World of Warcraft developer, Blizzard Entertainment, are expecting their next major release to be a free-to-play model. Blizzard are said to be developing the game, which is an upcoming, but unannounced game.
After eight years of phenomenal success with World of Warcraft, Blizzard are said to be expanding their business opportunities, and create both subscription and freemium titles. No other details are known of the game, apart from the potential free-to-play business model. Blizzard are currently working on "Project Titan" as well as polishing the soon-to-be-released Diablo III.
I'm sure we'll get some information on it soon enough, but knowing Blizzard it could be years before we actually see the product. It'll be an exciting time for the company, as it'll mark a huge change in the way they gain customers and interact with them, if the free-to-play game succeeds.