Zynga's social networking game Mafia Wars has hit four million users.
According to Gamasutra, daily players have doubled over the last three months, and like most free-to-play titles, it's micro-transaction based should you choose to purchase items.
To help build on the success of this genre, Zynga has hired game designer Brian Reynolds whose previous works include Alpha Centauri and Rise of Nations.
The browser-based multiplayer RPG is available on Myspace, Facebook, Yahoo Games and iPhone.
We had no idea how popular it was, despite the barrage of Facebook invitations we get for it in a single day. But there ya go.