The Steam Inventory Service is a brand new tool from Valve which feature allows developers to enable “persistent items that have been purchased or unlocked by individual users” without writing their own systems in support of this.
“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,” said Valve.
“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.”
It’s a nice addition to Steamworks which provides the biggest digital store for the PC developers around the world, achievements, DRM and lots of other features.