The first free-to-play games on Steam include Spiral Knights, Forsaken Worlds, Champions Online: Free for All, Global Agenda: Free Agent and Alliance of Valiant Arms.
Users can use their Steam wallet to pay for in-game content using the Steamworks backend system currently used by Team Fortress 2.
"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," said Jason Holtman, director of business development at Valve.
Valve is also running an "F2P game of the day" promotion during the next week, giving exclusive free in-game content to those who try that day's game. The promotion begins Wednesday with Spiral Knights and concludes on Sunday with Alliance of Valiant Arms.
The five initial free-to-play games are available now on Steam.