The service currently features 24 Linux games, a list dominated by indie titles that were already available via other means, as well as larger releases like Team Fortress 2 and Serious Sam 3. Valve has promised that internally developed titles like Portal and Left 4 Dead 2 will be available for Linux soon. Only one paid Steam Linux title, World of Goo, currently has a free demo available on the platform.
We shall see how this takes off.