Darkfall Unholy Wars is the first title to offer aforementioned service through Steam at $14.95 per month.
To buy a Subscription Plan, visit the subscription-based game's page on the Steam Store and select a Plan. Then, add it to your cart. A credit card is required to purchase Subscription Plans.
Steam users can edit/manage, renew or cancel their subscription but there is no option to "freeze," no credit will be given for any outstanding balance on your subscription, and there is no guarantee that when you renew you will be given access to your old game history. This depends on the game itself and may require you to contact the publisher to get your game linked to your Steam account again.
The Steam subscription plan does not allow you to gift a subscription to a friend nor does it let you register multiple game accounts to a single Steam account.