While some free-to-play games, like Global Agenda, have been available on Steam before, Valve is now making a larger push to bring F2P games to Steam-users. Starting yesterday, Sprial Knights, Forsaken Worlds, Champions Online: Free for All, Global Agenda: Free Agent and Alliance of Valliant Arms became available for download, and you can actually register for the games within Steam itself without having to mess around with registrations on the games' individual sites.
Microtransactions, that fund these games, will be available through Steam as well - something that can prove quite lucrative for both Valve and the companies running the games themselves.