Valve has rolled out the Steam Inventory Service in beta, allowing game developers the option to create in-game items that can be traded and sold throughout the greater Steam Marketplace.
A post on Steam News details the pre-release launch of the new feature set:
With this service, a game can easily drop items to customers based on playtime or can grant items based on specific situations or actions within the game. These items can be marked as tradable through Steam or sellable via the Steam Marketplace. Developers can also configure recipes for crafting different combinations of items that result in more rare, unique, or valuable items.”
"The Steam Inventory Service is a set of new Steamworks APIs and tools that allow a game to enable persistent items that have been purchase or unlocked by individual users without having to run special servers to keep track of these user's inventory."
In a nutshell, this means Valve's in-game item marketplace that makes lots of money for certain folks working with Valve-produced games is now open to any developer with a product on Steam.
The option is now included in the latest beta release of Valve's Steamworks service, which already boasts support for Steam Workshop, achievements, cloud saves, authentication services, error reporting, leaderboards, and matchmaking–among other features for devs to take advantage of.