StoryDefense of the Ancients started as a mod for Warcraft 3, so free as long as you owned the game. Then people decided to split off and make standalone versions. League of Legends came out and was free to play from the start. Heroes of Newerth had a long open beta and then was sold for $30 and is now switching to F2P with a similar model as LoL. Valve is making DOTA 2 and its been long rumored that it will be F2P as well.
Its interesting that it seems like an entire genre of games is locked into this business model. I guess when a genre only has a handful of games in it where a couple are free or really cheap you'll have a hell of a time convincing people to buy your game. Though it doesn't help that they are all so similar to each other, why pay when you can get basically the same thing for free?
