Valve Software announced that the very first game to make use of its freshly launched support for subscriptions is the MMO Darkfall Unholy Wars, with "additional subscription-based games to follow."
The Steam Subscription service allows game developers and publishers of all sizes to charge users recurring subscription service fees directly within Steam. Prior to the launch of this service, those services would have to be handled within the game's client or on the developer's website for the game. Now users can manage subscriptions without leaving the Steam Client.
Steam customers can "sign-up for, manage, cancel or renew game Subscription Plans at any time, online directly through Steam."
You can learn more about Steam Subscriptions at the service's freshly launched support page.