Steam has become the standard for downloadable games for PC / Mac / Linux. There are two issues – one, it doesn’t carry software that will run on Chrome OS, and two, it’s not available for Chrome OS. That’s not the end of the story for gaming though – and even better, in order to run games on Chrome OS you don’t have to download any software whatsoever.
The Chrome web store
The Chrome web store has a surprisingly decent selection of games, including stalwarts like Angry Birds, Bastion, Cut The Rope, and Runescape. There are plenty of simple puzzle games as you’d expect (e.g. Bejeweled, Fieldrunners) but you’ll also find games that run in 3D (Need for Speed, Hitman) and work offline.
Since Chrome OS runs flash, you also have access to any of the flash game sites on the web like Kongregate. These sites mainly provide casual gaming experiences, but there are some great and addictive finds in there.