It seemed that Valve was planning to make not one big announcement, but three. When the countdown clock at the bottom of the page reached "00:00:00" yesterday, Valve officially announced SteamOS.
SteamOS, according to the announcement page, is an operating system optimized for gaming and meant as a "free, stand-alone operating system for living room machines." Presumably, the operating system will be open-source and available for anyone to install on a system of their own, like Android.
There is now another countdown clock ticking down to zero on Steam's teaser page. The first announcement made today was represented with a "O" and the one that will occur 48 hours after the first announcement is represented by a "[O ]", with many speculating that the "O" symbol inside two brackets means that the next announcement will be the long-rumored SteamBox game console, referred to jokingly as GabeCube, a play on the GameCube console and Gabe Newell.