Another MMO has adopted the freemium model. Some two years after Star Trek Online went live for paying subscribers, the game has become free to play. You don't even need a credit card to sign up.
Players intent on opening their wallets can still opt for a "gold" subscription, which costs $15 per month or $300 for a lifetime. As one might expect, gold status imparts numerous benefits. Developer Cryptic Studios has a helpful feature matrix that outlines all the subscriber perks, and they look pretty reasonable to me. Free players still have access to all the games sectors, missions, classes, and events. Gold members get more player and inventory slots, though. They also have unlimited access to some functions that freeloaders will have to unlock by playing the game.
The freemium approach is becoming increasingly popular in the MMO world, and I suspect we'll see it branch out more in the coming years. A good litmus test for the model's merit in other genres will be first-person shooter Tribes Ascend, which will be free to play from day one. The game is currently in a closed beta accessible to paying subscribers, so it has its own set of premium perks. Whether those perks affect the gameplay balance remains to be seen.