Beginning tomorrow (Weds) with Spiral Knights and concluding Sunday with AVA, each of the five launch titles will offer exclusive in-game content to those who try the "F2P game of the day" on Steam.  In game transactions in all of the titles will be supported by Steam's micro-transaction backend system that is available to Steamworks partners and currently used in Valve's Team Fortress 2.

"The introduction of Free to Play games is another example of the constant evolution of Steam," said Jason Holtman, director of business development at Valve. "Free to Play games offer new game genres and game experiences for customers, while offering developers and publishers new revenue opportunities and the ability to reach customers in areas of the world where the traditional packaged goods model is less popular than F2P."

Specific details on the F2P games of the day and their exclusive in-game content will be made available in the coming days.