From wiki, regarding Steamplay:
Originally planned for release in April 2010, Steam for Mac OS X was launched worldwide on May 12, 2010, following a successful beta period. In addition to the Steam client, several features were made available to developers to take advantage of a cross-platform Source engine and platform and network capabilities using Steamworks. Through SteamPlay, the Mac OS X client will allow players who have already purchased compatible products in the Windows version to re-download the Mac versions at no cost, allowing them to continue to play the game on the other platform; however some third party titles may require the user to purchase again to gain the cross-platform functionality. The Steam Cloud will be cross-platform compatible. Multiplayer games can also be cross-compatible, allowing Windows and Mac players to play with each other.
Upon launch, over 50 games, most supporting the SteamPlay feature, were available for the client. As part of the launch, Valve offered both PC and Mac users to download Portal for free during the first two weeks of launch. For some weeks after the Mac client launch, Valve will expand the catalog of offerings for the service on a weekly basis, with each week highlighting a new feature of the service. Valve plans to include native Mac OS X; OpenGL versions of Left 4 Dead, Left 4 Dead 2, Team Fortress 2, Counter-Strike: Source, Portal, and the entire Half-Life series following Steam's release. Portal 2, due in 2011, will be the first Valve title simultaneously released on both the Windows and Mac versions of Steam.