According to a report on Develop, World of Warcraft and StarCraft developers Blizzard are working on "an upcoming, as yet unannounced game" which will be based around a free-to-play business model.
It's unknown whether this free-to-play (F2P) game is Blizzard's famous "Project Titan" or a separate title.
To date, Blizzard's games have either been available at retail (ala StarCraft) or via a monthly subscription (ala World of Warcraft).
UPDATE - Blizzard tells Kotaku that it is indeed working on a free-to-play title. It's called Blizzard DOTA, which is very much announced. Nothing to see here!